by David Hildenbrand

[permalink] [raw]

Subject: Re: [RFC PATCH 0/7] Memory hotplug/hotremove at subsection size

On 06.05.21 17:26, Zi Yan wrote:
> From: Zi Yan <[email protected]>
>
> Hi all,
>
> This patchset tries to remove the restriction on memory hotplug/hotremove
> granularity, which is always greater or equal to memory section size[1].
> With the patchset, kernel is able to online/offline memory at a size independent
> of memory section size, as small as 2MB (the subsection size).

... which doesn't make any sense as we can only online/offline whole
memory block devices.

>
> The motivation is to increase MAX_ORDER of the buddy allocator and pageblock
> size without increasing memory hotplug/hotremove granularity at the same time,

Gah, no. Please no. No.

> so that the kernel can allocator 1GB pages using buddy allocator and utilizes
> existing pageblock based anti-fragmentation, paving the road for 1GB THP
> support[2].

Not like this, please no.

>
> The patchset utilizes the existing subsection support[3] and changes the
> section size alignment checks to subsection size alignment checks. There are
> also changes to pageblock code to support partial pageblocks, when pageblock
> size is increased along with MAX_ORDER. Increasing pageblock size can enable
> kernel to utilize existing anti-fragmentation mechanism for gigantic page
> allocations.

Please not like this.

>
> The last patch increases SECTION_SIZE_BITS to demonstrate the use of memory
> hotplug/hotremove subsection, but is not intended to be merged as is. It is
> there in case one wants to try this out and will be removed during the final
> submission.
>
> Feel free to give suggestions and comments. I am looking forward to your
> feedback.

Please not like this.

--
Thanks,

David / dhildenb

2021-05-06 15:43:58

by Zi Yan

[permalink] [raw]

Subject: Re: [RFC PATCH 0/7] Memory hotplug/hotremove at subsection size

On 6 May 2021, at 11:26, Zi Yan wrote:

> From: Zi Yan <[email protected]>
>
> Hi all,
>
> This patchset tries to remove the restriction on memory hotplug/hotremove
> granularity, which is always greater or equal to memory section size[1].
> With the patchset, kernel is able to online/offline memory at a size independent
> of memory section size, as small as 2MB (the subsection size).
>
> The motivation is to increase MAX_ORDER of the buddy allocator and pageblock
> size without increasing memory hotplug/hotremove granularity at the same time,
> so that the kernel can allocator 1GB pages using buddy allocator and utilizes
> existing pageblock based anti-fragmentation, paving the road for 1GB THP
> support[2].
>
> The patchset utilizes the existing subsection support[3] and changes the
> section size alignment checks to subsection size alignment checks. There are
> also changes to pageblock code to support partial pageblocks, when pageblock
> size is increased along with MAX_ORDER. Increasing pageblock size can enable
> kernel to utilize existing anti-fragmentation mechanism for gigantic page
> allocations.
>
> The last patch increases SECTION_SIZE_BITS to demonstrate the use of memory
> hotplug/hotremove subsection, but is not intended to be merged as is. It is
> there in case one wants to try this out and will be removed during the final
> submission.
>
> Feel free to give suggestions and comments. I am looking forward to your
> feedback.
>
> Thanks.

Added the missing references.

[1] https://lore.kernel.org/linux-mm/[email protected]/
[2] https://lore.kernel.org/linux-mm/[email protected]/
[3] https://patchwork.kernel.org/project/linux-nvdimm/cover/156092349300.979959.17603710711957735135.stgit@dwillia2-desk3.amr.corp.intel.com/

>
> Zi Yan (7):
> mm: sparse: set/clear subsection bitmap when pages are
> onlined/offlined.
> mm: set pageblock_order to the max of HUGETLB_PAGE_ORDER and
> MAX_ORDER-1
> mm: memory_hotplug: decouple memory_block size with section size.
> mm: pageblock: allow set/unset migratetype for partial pageblock
> mm: memory_hotplug, sparse: enable memory hotplug/hotremove
> subsections
> arch: x86: no MAX_ORDER exceeds SECTION_SIZE check for 32bit vdso.
> [not for merge] mm: increase SECTION_SIZE_BITS to 31
>
> arch/ia64/Kconfig | 1 -
> arch/powerpc/Kconfig | 1 -
> arch/x86/Kconfig | 15 +++
> arch/x86/entry/vdso/Makefile | 1 +
> arch/x86/include/asm/sparsemem.h | 2 +-
> drivers/base/memory.c | 176 +++++++++++++++----------------
> drivers/base/node.c | 2 +-
> include/linux/memory.h | 8 +-
> include/linux/mmzone.h | 2 +
> include/linux/page-isolation.h | 8 +-
> include/linux/pageblock-flags.h | 9 --
> mm/Kconfig | 7 --
> mm/memory_hotplug.c | 22 ++--
> mm/page_alloc.c | 40 ++++---
> mm/page_isolation.c | 30 +++---
> mm/sparse.c | 55 ++++++++--
> 16 files changed, 219 insertions(+), 160 deletions(-)
>
> --
> 2.30.2

—
Best Regards,
Yan Zi

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2021-05-06 15:52:27

On 6 May 2021, at 11:31, David Hildenbrand wrote:

> On 06.05.21 17:26, Zi Yan wrote:
>> From: Zi Yan <[email protected]>
>>
>> Hi all,
>>
>> This patchset tries to remove the restriction on memory hotplug/hotremove
>> granularity, which is always greater or equal to memory section size[1].
>> With the patchset, kernel is able to online/offline memory at a size independent
>> of memory section size, as small as 2MB (the subsection size).
>
> ... which doesn't make any sense as we can only online/offline whole memory block devices.

Why limit the memory block size to section size? Patch 3 removes the restriction
by using (start_pfn, nr_pages) to allow memory block size goes below section size.
Also we have subsection bitmap available to tell us which subsection is online,
there is no reason to force memory block size to match section size.

>
>>
>> The motivation is to increase MAX_ORDER of the buddy allocator and pageblock
>> size without increasing memory hotplug/hotremove granularity at the same time,
>
> Gah, no. Please no. No.
>
>> so that the kernel can allocator 1GB pages using buddy allocator and utilizes
>> existing pageblock based anti-fragmentation, paving the road for 1GB THP
>> support[2].
>
> Not like this, please no.
>
>>
>> The patchset utilizes the existing subsection support[3] and changes the
>> section size alignment checks to subsection size alignment checks. There are
>> also changes to pageblock code to support partial pageblocks, when pageblock
>> size is increased along with MAX_ORDER. Increasing pageblock size can enable
>> kernel to utilize existing anti-fragmentation mechanism for gigantic page
>> allocations.
>
> Please not like this.
>
>>
>> The last patch increases SECTION_SIZE_BITS to demonstrate the use of memory
>> hotplug/hotremove subsection, but is not intended to be merged as is. It is
>> there in case one wants to try this out and will be removed during the final
>> submission.
>>
>> Feel free to give suggestions and comments. I am looking forward to your
>> feedback.
>
> Please not like this.

Do you mind sharing more useful feedback instead of just saying a lot of No?

Thanks.

—
Best Regards,
Yan Zi

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2021-05-06 19:27:01

by David Hildenbrand

[permalink] [raw]

Subject: Re: [RFC PATCH 0/7] Memory hotplug/hotremove at subsection size

On 06.05.21 17:31, David Hildenbrand wrote:
> On 06.05.21 17:26, Zi Yan wrote:
>> From: Zi Yan <[email protected]>
>>
>> Hi all,
>>
>> This patchset tries to remove the restriction on memory hotplug/hotremove
>> granularity, which is always greater or equal to memory section size[1].
>> With the patchset, kernel is able to online/offline memory at a size independent
>> of memory section size, as small as 2MB (the subsection size).
>
> ... which doesn't make any sense as we can only online/offline whole
> memory block devices.
>
>>
>> The motivation is to increase MAX_ORDER of the buddy allocator and pageblock
>> size without increasing memory hotplug/hotremove granularity at the same time,
>
> Gah, no. Please no. No.
>
>> so that the kernel can allocator 1GB pages using buddy allocator and utilizes
>> existing pageblock based anti-fragmentation, paving the road for 1GB THP
>> support[2].
>
> Not like this, please no.
>
>>
>> The patchset utilizes the existing subsection support[3] and changes the
>> section size alignment checks to subsection size alignment checks. There are
>> also changes to pageblock code to support partial pageblocks, when pageblock
>> size is increased along with MAX_ORDER. Increasing pageblock size can enable
>> kernel to utilize existing anti-fragmentation mechanism for gigantic page
>> allocations.
>
> Please not like this.
>
>>
>> The last patch increases SECTION_SIZE_BITS to demonstrate the use of memory
>> hotplug/hotremove subsection, but is not intended to be merged as is. It is
>> there in case one wants to try this out and will be removed during the final
>> submission.
>>
>> Feel free to give suggestions and comments. I am looking forward to your
>> feedback.
>
> Please not like this.
>

And just to be clear (I think I mentioned this already to you?): Nack to
increasing the section size. Nack to increasing the pageblock order.
Please find different ways to group on gigantic-pages level. There are
alternative ideas floating around.

Semi-nack to increasing MAX_ORDER. I first want to see
alloc_contig_range() be able to fully and cleanly handle allocations <
MAX_ORDER in all cases (especially !CMA and !ZONE_MOVABLE) before we go
down that path.

--
Thanks,

David / dhildenb

2021-05-06 19:35:48

by David Hildenbrand

[permalink] [raw]

Subject: Re: [RFC PATCH 0/7] Memory hotplug/hotremove at subsection size

On 06.05.21 17:50, Zi Yan wrote:
> On 6 May 2021, at 11:40, David Hildenbrand wrote:
>
>>>>> The last patch increases SECTION_SIZE_BITS to demonstrate the use of memory
>>>>> hotplug/hotremove subsection, but is not intended to be merged as is. It is
>>>>> there in case one wants to try this out and will be removed during the final
>>>>> submission.
>>>>>
>>>>> Feel free to give suggestions and comments. I am looking forward to your
>>>>> feedback.
>>>>
>>>> Please not like this.
>>>
>>> Do you mind sharing more useful feedback instead of just saying a lot of No?
>>
>> I remember reasoning about this already in another thread, no? Either you're ignoring my previous feedback or my mind is messing with me.
>
> I definitely remember all your suggestions:
>
> 1. do not use CMA allocation for 1GB THP.
> 2. section size defines the minimum size in which we can add_memory(), so we cannot increase it.
>
> I am trying an alternative here. I am not using CMA allocation and not increasing the minimum size of add_memory() by decoupling the memory block size from section size, so that add_memory() can add a memory block smaller (as small as 2MB, the subsection size) than section size. In this way, section size can be increased freely. I do not see the strong tie between add_memory() and section size, especially we have subsection bitmap support.

Okay, let me express my thoughts, I could have sworn I explained back
then why I am not a friend of messing with the existing pageblock size:

1. Pageblock size

There are a couple of features that rely on the pageblock size to be
reasonably small to work as expected. One example is virtio-balloon free
page reporting, then there is virtio-mem (still also glued MAX_ORDER)
and we have CMA (still also glued to MAX_ORDER). Most probably there are
more. We track movability/ page isolation per pageblock; it's the
smallest granularity you can effectively isolate pages or mark them as
CMA (MIGRATE_ISOLATE, MIGRATE_CMA). Well, and there are "ordinary" THP /
huge pages most of our applications use and will use, especially on
smallish systems.

Assume you bump up the pageblock order to 1G. Small VMs won't be able to
report any free pages to the hypervisor. You'll take the "fine-grained"
out of virtio-mem. Each CMA area will have to be at least 1G big, which
turns CMA essentially useless on smallish systems (like we have on arm64
with 64k base pages -- pageblock_size is 512MB and I hate it).

Then, imagine systems that have like 4G of main memory. By stopping
grouping at 2M and instead grouping at 1G you can very easily find
yourself in the system where all your 4 pageblocks are unmovable and you
essentially don't optimize for huge pages in that environment any more.

Long story short: we need a different mechanism on top and shall leave
the pageblock size untouched, it's too tightly integrated with page
isolation, ordinary THP, and CMA.

2. Section size

I assume the only reason you want to touch that is because
pageblock_size <= section_size, and I guess that's one of the reasons I
dislike it so much. Messing with the section size really only makes
sense when we want to manage metadata for larger granularity within a
section.

We allocate metadata per section. We mark whole sections
early/online/present/.... Yes, in case of vmemmap, we manage the memmap
in smaller granularity using the sub-section map, some kind of hack to
support some ZONE_DEVICE cases better.

Let's assume we introduce something new "gigapage_order", corresponding
to 1G. We could either decide to squeeze the metadata into sections,
having to increase the section size, or manage that metadata differently.

Managing it differently certainly makes the necessary changes easier.
Instead of adding more hacks into sections, rather manage that metadata
at differently place / in a different way.

See [1] for an alternative. Not necessarily what I would dream off, but
just to showcase that there might be alternative to group pages.

3. Grouping pages > pageblock_order

There are other approaches that would benefit from grouping at >
pageblock_order and having bigger MAX_ORDER. And that doesn't
necessarily mean to form gigantic pages only, we might want to group in
multiple granularity on a single system. Memory hot(un)plug is one
example, but also optimizing memory consumption by powering down DIMM
banks. Also, some architectures support differing huge page sizes
(aarch64) that could be improved without CMA. Why not have more than 2
THP sizes on these systems?

Ideally, we'd have a mechanism that tries grouping on different
granularity, like for every order in pageblock_order ...
max_pageblock_order (e.g., 1 GiB), and not only add one new level of
grouping (or increase the single grouping size).

[1] https://lkml.kernel.org/r/[email protected]

--
Thanks,

David / dhildenb

2021-05-06 19:55:27

On 6 May 2021, at 12:28, David Hildenbrand wrote:

> On 06.05.21 17:50, Zi Yan wrote:
>> On 6 May 2021, at 11:40, David Hildenbrand wrote:
>>
>>>>>> The last patch increases SECTION_SIZE_BITS to demonstrate the use of memory
>>>>>> hotplug/hotremove subsection, but is not intended to be merged as is. It is
>>>>>> there in case one wants to try this out and will be removed during the final
>>>>>> submission.
>>>>>>
>>>>>> Feel free to give suggestions and comments. I am looking forward to your
>>>>>> feedback.
>>>>>
>>>>> Please not like this.
>>>>
>>>> Do you mind sharing more useful feedback instead of just saying a lot of No?
>>>
>>> I remember reasoning about this already in another thread, no? Either you're ignoring my previous feedback or my mind is messing with me.
>>
>> I definitely remember all your suggestions:
>>
>> 1. do not use CMA allocation for 1GB THP.
>> 2. section size defines the minimum size in which we can add_memory(), so we cannot increase it.
>>
>> I am trying an alternative here. I am not using CMA allocation and not increasing the minimum size of add_memory() by decoupling the memory block size from section size, so that add_memory() can add a memory block smaller (as small as 2MB, the subsection size) than section size. In this way, section size can be increased freely. I do not see the strong tie between add_memory() and section size, especially we have subsection bitmap support.
>
> Okay, let me express my thoughts, I could have sworn I explained back then why I am not a friend of messing with the existing pageblock size:

Thanks for writing down your thoughts in detail. I will clarify my high-level plan below too.

>
> 1. Pageblock size
>
> There are a couple of features that rely on the pageblock size to be reasonably small to work as expected. One example is virtio-balloon free page reporting, then there is virtio-mem (still also glued MAX_ORDER) and we have CMA (still also glued to MAX_ORDER). Most probably there are more. We track movability/ page isolation per pageblock; it's the smallest granularity you can effectively isolate pages or mark them as CMA (MIGRATE_ISOLATE, MIGRATE_CMA). Well, and there are "ordinary" THP / huge pages most of our applications use and will use, especially on smallish systems.
>
> Assume you bump up the pageblock order to 1G. Small VMs won't be able to report any free pages to the hypervisor. You'll take the "fine-grained" out of virtio-mem. Each CMA area will have to be at least 1G big, which turns CMA essentially useless on smallish systems (like we have on arm64 with 64k base pages -- pageblock_size is 512MB and I hate it).

I understand the issue of having large pageblock in small systems. My plan for this issue is to make MAX_ORDER a variable (pageblock size would be set according to MAX_ORDER) that can be adjusted based on total memory and via boot time parameter. My apology since I did not state this clearly in my cover letter and it confused you. When we have a boot time adjustable MAX_ORDER, large pageblock like 1GB would only appear for systems with large memory. For small VMs, pageblock size would stay at 2MB, so all your concerns on smallish systems should go away.

>
> Then, imagine systems that have like 4G of main memory. By stopping grouping at 2M and instead grouping at 1G you can very easily find yourself in the system where all your 4 pageblocks are unmovable and you essentially don't optimize for huge pages in that environment any more.
>
> Long story short: we need a different mechanism on top and shall leave the pageblock size untouched, it's too tightly integrated with page isolation, ordinary THP, and CMA.

I think it is better to make pageblock size adjustable based on total memory of a system. It is not reasonable to have the same pageblock size across systems with memory sizes from <1GB to several TBs. Do you agree?

>
> 2. Section size
>
> I assume the only reason you want to touch that is because pageblock_size <= section_size, and I guess that's one of the reasons I dislike it so much. Messing with the section size really only makes sense when we want to manage metadata for larger granularity within a section.

Perhaps it is worth checking if it is feasible to make pageblock_size > section_size, so we can still have small sections when pageblock_size are large. One potential issue for that is when PFN discontinues at section boundary, we might have partial pageblock when pageblock_size is big. I guess supporting partial pageblock (or different pageblock sizes like you mentioned below ) would be the right solution.

>
> We allocate metadata per section. We mark whole sections early/online/present/.... Yes, in case of vmemmap, we manage the memmap in smaller granularity using the sub-section map, some kind of hack to support some ZONE_DEVICE cases better.
>
> Let's assume we introduce something new "gigapage_order", corresponding to 1G. We could either decide to squeeze the metadata into sections, having to increase the section size, or manage that metadata differently.
>
> Managing it differently certainly makes the necessary changes easier. Instead of adding more hacks into sections, rather manage that metadata at differently place / in a different way.

Can you elaborate on managing it differently?

>
> See [1] for an alternative. Not necessarily what I would dream off, but just to showcase that there might be alternative to group pages.

I saw this patch too. It is an interesting idea to separate different allocation orders into different regions, but it would not work for gigantic page allocations unless we have large pageblock size to utilize existing anti-fragmentation mechanisms.

>
> 3. Grouping pages > pageblock_order
>
> There are other approaches that would benefit from grouping at > pageblock_order and having bigger MAX_ORDER. And that doesn't necessarily mean to form gigantic pages only, we might want to group in multiple granularity on a single system. Memory hot(un)plug is one example, but also optimizing memory consumption by powering down DIMM banks. Also, some architectures support differing huge page sizes (aarch64) that could be improved without CMA. Why not have more than 2 THP sizes on these systems?
>
> Ideally, we'd have a mechanism that tries grouping on different granularity, like for every order in pageblock_order ... max_pageblock_order (e.g., 1 GiB), and not only add one new level of grouping (or increase the single grouping size).

I agree. In some sense, supporting partial pageblock and increasing pageblock size (e.g., to 1GB) is, at the high level, quite similar to having multiple pageblock sizes. But I am not sure if we really want to support multiple pageblock sizes, since it creates pageblock fragmentation when we want to change migratetype for part of a pageblock. This means we would break a large pageblock into small ones if we just want to steal a subset of pages from MOVEABLE for UNMOVABLE allocations. Then pageblock loses its most useful anti-fragmentation feature. Also it seems to be a replication of buddy allocator functionalities when it comes to pageblock split and merge.

The above is really a nice discussion with you on pageblock, section, memory hotplug/hotremove, which also helps me understand more on the issues with increasing MAX_ORDER to enable 1GB page allocation.

In sum, if I get it correctly, the issues I need to address are:

1. large pageblock size (which is needed when we bump MAX_ORDER for gigantic page allocation from buddy allocator) is not good for machines with small memory.

2. pageblock size is currently tied with section size (which made me want to bump section size).

For 1, I think making MAX_ORDER a variable that can be set based on total memory size and adjustable via boot time parameter should solve the problem. For small machines, we will keep MAX_ORDER as small as we have now like 4MB, whereas for large machines, we can increase MAX_ORDER to utilize gigantic pages.

For 2, supporting partial pageblock and allow a pageblock to cross multiple sections would break the tie between pageblock size and section to solve the issue.

I am going to look into them. What do you think?

—
Best Regards,
Yan Zi

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2021-05-07 15:12:01

by Michal Hocko

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Subject: Re: [RFC PATCH 0/7] Memory hotplug/hotremove at subsection size

[I haven't read through respective patches due to lack of time but let
me comment on the general idea and the underlying justification]

On Thu 06-05-21 17:31:09, David Hildenbrand wrote:
> On 06.05.21 17:26, Zi Yan wrote:
> > From: Zi Yan <[email protected]>
> >
> > Hi all,
> >
> > This patchset tries to remove the restriction on memory hotplug/hotremove
> > granularity, which is always greater or equal to memory section size[1].
> > With the patchset, kernel is able to online/offline memory at a size independent
> > of memory section size, as small as 2MB (the subsection size).
>
> ... which doesn't make any sense as we can only online/offline whole memory
> block devices.

Agreed. The subsection thingy is just a hack to workaround pmem
alignement problems. For the real memory hotplug it is quite hard to
argue for reasonable hotplug scenarios for very small physical memory
ranges wrt. to the existing sparsemem memory model.

> > The motivation is to increase MAX_ORDER of the buddy allocator and pageblock
> > size without increasing memory hotplug/hotremove granularity at the same time,
>
> Gah, no. Please no. No.

Agreed. Those are completely independent concepts. MAX_ORDER is can be
really arbitrary irrespective of the section size with vmemmap sparse
model. The existing restriction is due to old sparse model not being
able to do page pointer arithmetic across memory sections. Is there any
reason to stick with that memory model for an advance feature you are
working on?
--
Michal Hocko
SUSE Labs

2021-05-07 17:16:46

by David Hildenbrand

[permalink] [raw]

Subject: Re: [RFC PATCH 0/7] Memory hotplug/hotremove at subsection size

On 07.05.21 13:55, Michal Hocko wrote:
> [I haven't read through respective patches due to lack of time but let
> me comment on the general idea and the underlying justification]
>
> On Thu 06-05-21 17:31:09, David Hildenbrand wrote:
>> On 06.05.21 17:26, Zi Yan wrote:
>>> From: Zi Yan <[email protected]>
>>>
>>> Hi all,
>>>
>>> This patchset tries to remove the restriction on memory hotplug/hotremove
>>> granularity, which is always greater or equal to memory section size[1].
>>> With the patchset, kernel is able to online/offline memory at a size independent
>>> of memory section size, as small as 2MB (the subsection size).
>>
>> ... which doesn't make any sense as we can only online/offline whole memory
>> block devices.
>
> Agreed. The subsection thingy is just a hack to workaround pmem
> alignement problems. For the real memory hotplug it is quite hard to
> argue for reasonable hotplug scenarios for very small physical memory
> ranges wrt. to the existing sparsemem memory model.
>
>>> The motivation is to increase MAX_ORDER of the buddy allocator and pageblock
>>> size without increasing memory hotplug/hotremove granularity at the same time,
>>
>> Gah, no. Please no. No.
>
> Agreed. Those are completely independent concepts. MAX_ORDER is can be
> really arbitrary irrespective of the section size with vmemmap sparse
> model. The existing restriction is due to old sparse model not being
> able to do page pointer arithmetic across memory sections. Is there any
> reason to stick with that memory model for an advance feature you are
> working on?
>

I gave it some more thought yesterday. I guess the first thing we should
look into is increasing MAX_ORDER and leaving pageblock_order and
section size as is -- finding out what we have to tweak to get that up
and running. Once we have that in place, we can actually look into
better fragmentation avoidance etc. One step at a time.

Because that change itself might require some thought. Requiring that
bigger MAX_ORDER depends on SPARSE_VMEMMAP is something reasonable to do.

As stated somewhere here already, we'll have to look into making
alloc_contig_range() (and main users CMA and virtio-mem) independent of
MAX_ORDER and mainly rely on pageblock_order. The current handling in
alloc_contig_range() is far from optimal as we have to isolate a whole
MAX_ORDER - 1 page -- and on ZONE_NORMAL we'll fail easily if any part
contains something unmovable although we don't even want to allocate
that part. I actually have that on my list (to be able to fully support
pageblock_order instead of MAX_ORDER -1 chunks in virtio-mem), however
didn't have time to look into it.

Further, page onlining / offlining code and early init code most
probably also needs care if MAX_ORDER - 1 crosses sections. Memory holes
we might suddenly have in MAX_ORDER - 1 pages might become a problem and
will have to be handled. Not sure which other code has to be tweaked
(compaction? page isolation?).

Figuring out what needs care itself might take quite some effort.

One thing I was thinking about as well: The bigger our MAX_ORDER, the
slower it could be to allocate smaller pages. If we have 1G pages,
splitting them down to 4k then takes 8 additional steps if I'm, not
wrong. Of course, that's the worst case. Would be interesting to evaluate.

--
Thanks,

David / dhildenb

2021-05-10 14:42:04

On 10 May 2021, at 10:36, Zi Yan wrote:

> On 7 May 2021, at 10:00, David Hildenbrand wrote:
>
>> On 07.05.21 13:55, Michal Hocko wrote:
>>> [I haven't read through respective patches due to lack of time but let
>>> me comment on the general idea and the underlying justification]
>>>
>>> On Thu 06-05-21 17:31:09, David Hildenbrand wrote:
>>>> On 06.05.21 17:26, Zi Yan wrote:
>>>>> From: Zi Yan <[email protected]>
>>>>>
>>>>> Hi all,
>>>>>
>>>>> This patchset tries to remove the restriction on memory hotplug/hotremove
>>>>> granularity, which is always greater or equal to memory section size[1].
>>>>> With the patchset, kernel is able to online/offline memory at a size independent
>>>>> of memory section size, as small as 2MB (the subsection size).
>>>>
>>>> ... which doesn't make any sense as we can only online/offline whole memory
>>>> block devices.
>>>
>>> Agreed. The subsection thingy is just a hack to workaround pmem
>>> alignement problems. For the real memory hotplug it is quite hard to
>>> argue for reasonable hotplug scenarios for very small physical memory
>>> ranges wrt. to the existing sparsemem memory model.
>>>
>>>>> The motivation is to increase MAX_ORDER of the buddy allocator and pageblock
>>>>> size without increasing memory hotplug/hotremove granularity at the same time,
>>>>
>>>> Gah, no. Please no. No.
>>>
>>> Agreed. Those are completely independent concepts. MAX_ORDER is can be
>>> really arbitrary irrespective of the section size with vmemmap sparse
>>> model. The existing restriction is due to old sparse model not being
>>> able to do page pointer arithmetic across memory sections. Is there any
>>> reason to stick with that memory model for an advance feature you are
>>> working on?
>
> No. I just want to increase MAX_ORDER. If the existing restriction can
> be removed, that will be great.
>
>>
>> I gave it some more thought yesterday. I guess the first thing we should look into is increasing MAX_ORDER and leaving pageblock_order and section size as is -- finding out what we have to tweak to get that up and running. Once we have that in place, we can actually look into better fragmentation avoidance etc. One step at a time.
>
> It makes sense to me.
>
>>
>> Because that change itself might require some thought. Requiring that bigger MAX_ORDER depends on SPARSE_VMEMMAP is something reasonable to do.
>
> OK, if with SPARSE_VMEMMAP MAX_ORDER can be set to be bigger than
> SECTION_SIZE, it is perfectly OK to me. Since 1GB THP support, which I
> want to add ultimately, will require SPARSE_VMEMMAP too (otherwise,
> all page++ will need to be changed to nth_page(page,1)).
>
>>
>> As stated somewhere here already, we'll have to look into making alloc_contig_range() (and main users CMA and virtio-mem) independent of MAX_ORDER and mainly rely on pageblock_order. The current handling in alloc_contig_range() is far from optimal as we have to isolate a whole MAX_ORDER - 1 page -- and on ZONE_NORMAL we'll fail easily if any part contains something unmovable although we don't even want to allocate that part. I actually have that on my list (to be able to fully support pageblock_order instead of MAX_ORDER -1 chunks in virtio-mem), however didn't have time to look into it.
>
> So in your mind, for gigantic page allocation (> MAX_ORDER), alloc_contig_range()
> should be used instead of buddy allocator while pageblock_order is kept at a small
> granularity like 2MB. Is that the case? Isn’t it going to have high fail rate
> when any of the pageblocks within a gigantic page range (like 1GB) becomes unmovable?
> Are you thinking additional mechanism/policy to prevent such thing happening as
> an additional step for gigantic page allocation? Like your ZONE_PREFER_MOVABLE idea?
>
>>
>> Further, page onlining / offlining code and early init code most probably also needs care if MAX_ORDER - 1 crosses sections. Memory holes we might suddenly have in MAX_ORDER - 1 pages might become a problem and will have to be handled. Not sure which other code has to be tweaked (compaction? page isolation?).
>
> Can you elaborate it a little more? From what I understand, memory holes mean valid
> PFNs are not contiguous before and after a hole, so pfn++ will not work, but
> struct pages are still virtually contiguous assuming SPARSE_VMEMMAP, meaning page++
> would still work. So when MAX_ORDER - 1 crosses sections, additional code would be
> needed instead of simple pfn++. Is there anything I am missing?
>
> BTW, to test a system with memory holes, do you know is there an easy of adding
> random memory holes to an x86_64 VM, which can help reveal potential missing pieces
> in the code? Changing BIOS-e820 table might be one way, but I have no idea on
> how to do it on QEMU.
>
>>
>> Figuring out what needs care itself might take quite some effort.
>>
>> One thing I was thinking about as well: The bigger our MAX_ORDER, the slower it could be to allocate smaller pages. If we have 1G pages, splitting them down to 4k then takes 8 additional steps if I'm, not wrong. Of course, that's the worst case. Would be interesting to evaluate.
>
> Sure. I am planning to check it too. As a simple start, I am going to run will it scale
> benchmarks to see if there is any performance difference between different MAX_ORDERs.

I ran vm-scalablity and memory-related will-it-scale on a server with 256GB memory to
see the impact of increasing MAX_ORDER and didn’t see much difference for most of
the workloads like page_fault1, page_fault2, and page_fault3 from will-it-scale.
But feel free to check the attached complete results and let me know what should be
looked into. Thanks.

# Environment
Dell R630 with 2x 12-core E5-2650 v4 and 256GB memory.

# Kernel changes
On top of v5.13-rc1-mmotm-2021-05-13-17-18, with SECTION_SIZE_BITS set to 31 and
MAX_ORDER set to 11 and 20 respectively.

# Results of page_fault1, page_fault2, and page_fault3

compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/50%/debian/dellr630/page_fault3/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
3199850 ± 2% +6.0% 3390866 ± 3% will-it-scale.24.threads
54.94 +1.7% 55.85 will-it-scale.24.threads_idle
133326 ± 2% +6.0% 141285 ± 3% will-it-scale.per_thread_ops
3199850 ± 2% +6.0% 3390866 ± 3% will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/50%/debian/dellr630/page_fault2/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
2016984 -6.6% 1883075 ± 2% will-it-scale.24.threads
69.69 -4.4% 66.64 will-it-scale.24.threads_idle
84040 -6.6% 78461 ± 2% will-it-scale.per_thread_ops
2016984 -6.6% 1883075 ± 2% will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/50%/debian/dellr630/page_fault1/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
2138067 -1.3% 2109865 will-it-scale.24.threads
63.34 +1.1% 64.06 will-it-scale.24.threads_idle
89085 -1.3% 87910 will-it-scale.per_thread_ops
2138067 -1.3% 2109865 will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/16/debian/dellr630/page_fault3/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
3216287 ± 3% +4.8% 3369356 ± 10% will-it-scale.16.threads
69.18 +0.5% 69.51 will-it-scale.16.threads_idle
201017 ± 3% +4.8% 210584 ± 10% will-it-scale.per_thread_ops
3216287 ± 3% +4.8% 3369356 ± 10% will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/16/debian/dellr630/page_fault2/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
2005510 -2.7% 1950620 ± 2% will-it-scale.16.threads
78.77 -0.2% 78.64 will-it-scale.16.threads_idle
125344 -2.7% 121913 ± 2% will-it-scale.per_thread_ops
2005510 -2.7% 1950620 ± 2% will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/16/debian/dellr630/page_fault1/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
2332446 -6.5% 2179823 ± 2% will-it-scale.16.threads
77.57 -2.0% 76.03 will-it-scale.16.threads_idle
145777 -6.5% 136238 ± 2% will-it-scale.per_thread_ops
2332446 -6.5% 2179823 ± 2% will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/100%/debian/dellr630/page_fault3/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
3236057 ± 2% -4.5% 3089222 ± 4% will-it-scale.48.threads
24.64 ± 7% -3.3% 23.83 ± 2% will-it-scale.48.threads_idle
67417 ± 2% -4.5% 64358 ± 4% will-it-scale.per_thread_ops
3236057 ± 2% -4.5% 3089222 ± 4% will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/100%/debian/dellr630/page_fault2/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
1611363 -0.1% 1609891 will-it-scale.48.threads
47.42 ± 2% +1.2% 48.01 will-it-scale.48.threads_idle
33569 -0.1% 33539 will-it-scale.per_thread_ops
1611363 -0.1% 1609891 will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/thread/100%/debian/dellr630/page_fault1/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
1776494 ± 3% -2.6% 1730693 will-it-scale.48.threads
43.36 ± 4% +0.5% 43.57 ± 2% will-it-scale.48.threads_idle
37010 ± 3% -2.6% 36055 will-it-scale.per_thread_ops
1776494 ± 3% -2.6% 1730693 will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/50%/debian/dellr630/page_fault3/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
15235214 -0.3% 15185167 will-it-scale.24.processes
49.63 -0.4% 49.45 will-it-scale.24.processes_idle
634800 -0.3% 632715 will-it-scale.per_process_ops
15235214 -0.3% 15185167 will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/50%/debian/dellr630/page_fault2/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
6700813 -0.6% 6662570 will-it-scale.24.processes
49.17 +0.0% 49.18 will-it-scale.24.processes_idle
279200 -0.6% 277606 will-it-scale.per_process_ops
6700813 -0.6% 6662570 will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/50%/debian/dellr630/page_fault1/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
8052059 -1.2% 7952172 will-it-scale.24.processes
49.48 -0.4% 49.29 will-it-scale.24.processes_idle
335502 -1.2% 331340 will-it-scale.per_process_ops
8052059 -1.2% 7952172 will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/16/debian/dellr630/page_fault3/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
10152559 +0.7% 10221240 will-it-scale.16.processes
66.10 -0.0% 66.09 will-it-scale.16.processes_idle
634534 +0.7% 638827 will-it-scale.per_process_ops
10152559 +0.7% 10221240 will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/16/debian/dellr630/page_fault2/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
4621434 -1.0% 4576959 will-it-scale.16.processes
66.14 -0.2% 65.98 will-it-scale.16.processes_idle
288839 -1.0% 286059 will-it-scale.per_process_ops
4621434 -1.0% 4576959 will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/16/debian/dellr630/page_fault1/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
5546153 -1.3% 5474778 will-it-scale.16.processes
66.02 -0.1% 65.98 will-it-scale.16.processes_idle
346634 -1.3% 342173 will-it-scale.per_process_ops
5546153 -1.3% 5474778 will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/100%/debian/dellr630/page_fault3/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
20575719 +0.4% 20651992 will-it-scale.48.processes
0.06 +5.6% 0.06 ± 7% will-it-scale.48.processes_idle
428660 +0.4% 430249 will-it-scale.per_process_ops
20575719 +0.4% 20651992 will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/100%/debian/dellr630/page_fault2/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
6984071 -1.1% 6906022 will-it-scale.48.processes
0.07 +4.8% 0.07 ± 6% will-it-scale.48.processes_idle
145501 -1.1% 143875 will-it-scale.per_process_ops
6984071 -1.1% 6906022 will-it-scale.workload

=========================================================================================
compiler/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
gcc-10/defconfig/process/100%/debian/dellr630/page_fault1/will-it-scale

commit:
5.13.0-rc1-mm1-max-order-11+
5.13.0-rc1-mm1-max-order-20+

5.13.0-rc1-mm1-m 5.13.0-rc1-mm1-max-order-20
---------------- ---------------------------
%stddev %change %stddev
\ | \
7527654 -1.7% 7399284 will-it-scale.48.processes
0.07 +0.0% 0.07 will-it-scale.48.processes_idle
156826 -1.7% 154151 will-it-scale.per_process_ops
7527654 -1.7% 7399284 will-it-scale.workload

—
Best Regards,
Yan, Zi

Attachments:

dellr630_2NUMA_24core_256GB_full_results.txt (68.74 kB)
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2021-06-14 11:43:37

by David Hildenbrand

[permalink] [raw]

Subject: Re: [RFC PATCH 0/7] Memory hotplug/hotremove at subsection size

On 02.06.21 17:56, Zi Yan wrote:
> On 10 May 2021, at 10:36, Zi Yan wrote:
>
>> On 7 May 2021, at 10:00, David Hildenbrand wrote:
>>
>>> On 07.05.21 13:55, Michal Hocko wrote:
>>>> [I haven't read through respective patches due to lack of time but let
>>>> me comment on the general idea and the underlying justification]
>>>>
>>>> On Thu 06-05-21 17:31:09, David Hildenbrand wrote:
>>>>> On 06.05.21 17:26, Zi Yan wrote:
>>>>>> From: Zi Yan <[email protected]>
>>>>>>
>>>>>> Hi all,
>>>>>>
>>>>>> This patchset tries to remove the restriction on memory hotplug/hotremove
>>>>>> granularity, which is always greater or equal to memory section size[1].
>>>>>> With the patchset, kernel is able to online/offline memory at a size independent
>>>>>> of memory section size, as small as 2MB (the subsection size).
>>>>>
>>>>> ... which doesn't make any sense as we can only online/offline whole memory
>>>>> block devices.
>>>>
>>>> Agreed. The subsection thingy is just a hack to workaround pmem
>>>> alignement problems. For the real memory hotplug it is quite hard to
>>>> argue for reasonable hotplug scenarios for very small physical memory
>>>> ranges wrt. to the existing sparsemem memory model.
>>>>
>>>>>> The motivation is to increase MAX_ORDER of the buddy allocator and pageblock
>>>>>> size without increasing memory hotplug/hotremove granularity at the same time,
>>>>>
>>>>> Gah, no. Please no. No.
>>>>
>>>> Agreed. Those are completely independent concepts. MAX_ORDER is can be
>>>> really arbitrary irrespective of the section size with vmemmap sparse
>>>> model. The existing restriction is due to old sparse model not being
>>>> able to do page pointer arithmetic across memory sections. Is there any
>>>> reason to stick with that memory model for an advance feature you are
>>>> working on?
>>
>> No. I just want to increase MAX_ORDER. If the existing restriction can
>> be removed, that will be great.
>>
>>>
>>> I gave it some more thought yesterday. I guess the first thing we should look into is increasing MAX_ORDER and leaving pageblock_order and section size as is -- finding out what we have to tweak to get that up and running. Once we have that in place, we can actually look into better fragmentation avoidance etc. One step at a time.
>>
>> It makes sense to me.
>>
>>>
>>> Because that change itself might require some thought. Requiring that bigger MAX_ORDER depends on SPARSE_VMEMMAP is something reasonable to do.
>>
>> OK, if with SPARSE_VMEMMAP MAX_ORDER can be set to be bigger than
>> SECTION_SIZE, it is perfectly OK to me. Since 1GB THP support, which I
>> want to add ultimately, will require SPARSE_VMEMMAP too (otherwise,
>> all page++ will need to be changed to nth_page(page,1)).
>>
>>>
>>> As stated somewhere here already, we'll have to look into making alloc_contig_range() (and main users CMA and virtio-mem) independent of MAX_ORDER and mainly rely on pageblock_order. The current handling in alloc_contig_range() is far from optimal as we have to isolate a whole MAX_ORDER - 1 page -- and on ZONE_NORMAL we'll fail easily if any part contains something unmovable although we don't even want to allocate that part. I actually have that on my list (to be able to fully support pageblock_order instead of MAX_ORDER -1 chunks in virtio-mem), however didn't have time to look into it.
>>
>> So in your mind, for gigantic page allocation (> MAX_ORDER), alloc_contig_range()
>> should be used instead of buddy allocator while pageblock_order is kept at a small
>> granularity like 2MB. Is that the case? Isn’t it going to have high fail rate
>> when any of the pageblocks within a gigantic page range (like 1GB) becomes unmovable?
>> Are you thinking additional mechanism/policy to prevent such thing happening as
>> an additional step for gigantic page allocation? Like your ZONE_PREFER_MOVABLE idea?
>>
>>>
>>> Further, page onlining / offlining code and early init code most probably also needs care if MAX_ORDER - 1 crosses sections. Memory holes we might suddenly have in MAX_ORDER - 1 pages might become a problem and will have to be handled. Not sure which other code has to be tweaked (compaction? page isolation?).
>>
>> Can you elaborate it a little more? From what I understand, memory holes mean valid
>> PFNs are not contiguous before and after a hole, so pfn++ will not work, but
>> struct pages are still virtually contiguous assuming SPARSE_VMEMMAP, meaning page++
>> would still work. So when MAX_ORDER - 1 crosses sections, additional code would be
>> needed instead of simple pfn++. Is there anything I am missing?
>>
>> BTW, to test a system with memory holes, do you know is there an easy of adding
>> random memory holes to an x86_64 VM, which can help reveal potential missing pieces
>> in the code? Changing BIOS-e820 table might be one way, but I have no idea on
>> how to do it on QEMU.
>>
>>>
>>> Figuring out what needs care itself might take quite some effort.
>>>
>>> One thing I was thinking about as well: The bigger our MAX_ORDER, the slower it could be to allocate smaller pages. If we have 1G pages, splitting them down to 4k then takes 8 additional steps if I'm, not wrong. Of course, that's the worst case. Would be interesting to evaluate.
>>
>> Sure. I am planning to check it too. As a simple start, I am going to run will it scale
>> benchmarks to see if there is any performance difference between different MAX_ORDERs.
>
> I ran vm-scalablity and memory-related will-it-scale on a server with 256GB memory to
> see the impact of increasing MAX_ORDER and didn’t see much difference for most of
> the workloads like page_fault1, page_fault2, and page_fault3 from will-it-scale.
> But feel free to check the attached complete results and let me know what should be
> looked into. Thanks.

Right, for will-it-scale it looks like there are mostly minor
differences, although I am not sure if the results are really stable
(reaching from -6% to +6%). For vm-scalability the numbers seem to vary
even more (e.g., stddev of ± 63%), so I have no idea how expressive they
are. But I guess for these benchmarks, the net change won't really be
significant.

Thanks!

--
Thanks,

David / dhildenb