Each `blkif` has a free pages pool for the grant mapping. The size of
the pool starts from zero and be increased on demand while processing
the I/O requests. If current I/O requests handling is finished or 100
milliseconds has passed since last I/O requests handling, it checks and
shrinks the pool to not exceed the size limit, `max_buffer_pages`.
Therefore, `blkfront` running guests can cause a memory pressure in the
`blkback` running guest by attaching a large number of block devices and
inducing I/O. System administrators can avoid such problematic
situations by limiting the maximum number of devices each guest can
attach. However, finding the optimal limit is not so easy. Improper
set of the limit can results in the memory pressure or a resource
underutilization. This commit avoids such problematic situations by
squeezing the pools (returns every free page in the pool to the system)
for a while (users can set this duration via a module parameter) if a
memory pressure is detected.
Base Version
------------
This patch is based on v5.4. A complete tree is also available at my
public git repo:
https://github.com/sjp38/linux/tree/blkback_aggressive_shrinking_v3
Patch History
-------------
Changes from v2 (https://lore.kernel.org/linux-block/[email protected])
- Rename the module parameter and variables for brevity (aggressive
shrinking -> squeezing)
Changes from v1 (https://lore.kernel.org/xen-devel/[email protected]/)
- Adjust the description to not use the term, `arbitrarily` (suggested
by Paul Durrant)
- Specify time unit of the duration in the parameter description,
(suggested by Maximilian Heyne)
- Change default aggressive shrinking duration from 1ms to 10ms
- Merge two patches into one single patch
SeongJae Park (1):
xen/blkback: Squeeze page pools if a memory pressure is detected
drivers/block/xen-blkback/blkback.c | 35 +++++++++++++++++++++++++++--
1 file changed, 33 insertions(+), 2 deletions(-)
--
2.17.1
On 09.12.19 09:58, SeongJae Park wrote:
> Each `blkif` has a free pages pool for the grant mapping. The size of
> the pool starts from zero and be increased on demand while processing
> the I/O requests. If current I/O requests handling is finished or 100
> milliseconds has passed since last I/O requests handling, it checks and
> shrinks the pool to not exceed the size limit, `max_buffer_pages`.
>
> Therefore, `blkfront` running guests can cause a memory pressure in the
> `blkback` running guest by attaching a large number of block devices and
> inducing I/O.
I'm having problems to understand how a guest can attach a large number
of block devices without those having been configured by the host admin
before.
If those devices have been configured, dom0 should be ready for that
number of devices, e.g. by having enough spare memory area for ballooned
pages.
So either I'm missing something here or your reasoning for the need of
the patch is wrong.
Juergen
> -----Original Message-----
> From: Jürgen Groß <[email protected]>
> Sent: 09 December 2019 09:39
> To: Park, Seongjae <[email protected]>; [email protected];
> [email protected]; [email protected]
> Cc: [email protected]; [email protected]; Durrant,
> Paul <[email protected]>; [email protected]; xen-
> [email protected]
> Subject: Re: [PATCH v3 0/1] xen/blkback: Squeeze page pools if a memory
> pressure
>
> On 09.12.19 09:58, SeongJae Park wrote:
> > Each `blkif` has a free pages pool for the grant mapping. The size of
> > the pool starts from zero and be increased on demand while processing
> > the I/O requests. If current I/O requests handling is finished or 100
> > milliseconds has passed since last I/O requests handling, it checks and
> > shrinks the pool to not exceed the size limit, `max_buffer_pages`.
> >
> > Therefore, `blkfront` running guests can cause a memory pressure in the
> > `blkback` running guest by attaching a large number of block devices and
> > inducing I/O.
>
> I'm having problems to understand how a guest can attach a large number
> of block devices without those having been configured by the host admin
> before.
>
> If those devices have been configured, dom0 should be ready for that
> number of devices, e.g. by having enough spare memory area for ballooned
> pages.
>
> So either I'm missing something here or your reasoning for the need of
> the patch is wrong.
>
I think the underlying issue is that persistent grant support is hogging memory in the backends, thereby compromising scalability. IIUC this patch is essentially a band-aid to get back to the scalability that was possible before persistent grant support was added. Ultimately the right answer should be to get rid of persistent grants support and use grant copy, but such a change is clearly more invasive and would need far more testing.
Paul
On 09.12.19 10:46, Durrant, Paul wrote:
>> -----Original Message-----
>> From: Jürgen Groß <[email protected]>
>> Sent: 09 December 2019 09:39
>> To: Park, Seongjae <[email protected]>; [email protected];
>> [email protected]; [email protected]
>> Cc: [email protected]; [email protected]; Durrant,
>> Paul <[email protected]>; [email protected]; xen-
>> [email protected]
>> Subject: Re: [PATCH v3 0/1] xen/blkback: Squeeze page pools if a memory
>> pressure
>>
>> On 09.12.19 09:58, SeongJae Park wrote:
>>> Each `blkif` has a free pages pool for the grant mapping. The size of
>>> the pool starts from zero and be increased on demand while processing
>>> the I/O requests. If current I/O requests handling is finished or 100
>>> milliseconds has passed since last I/O requests handling, it checks and
>>> shrinks the pool to not exceed the size limit, `max_buffer_pages`.
>>>
>>> Therefore, `blkfront` running guests can cause a memory pressure in the
>>> `blkback` running guest by attaching a large number of block devices and
>>> inducing I/O.
>>
>> I'm having problems to understand how a guest can attach a large number
>> of block devices without those having been configured by the host admin
>> before.
>>
>> If those devices have been configured, dom0 should be ready for that
>> number of devices, e.g. by having enough spare memory area for ballooned
>> pages.
>>
>> So either I'm missing something here or your reasoning for the need of
>> the patch is wrong.
>>
>
> I think the underlying issue is that persistent grant support is hogging memory in the backends, thereby compromising scalability. IIUC this patch is essentially a band-aid to get back to the scalability that was possible before persistent grant support was added. Ultimately the right answer should be to get rid of persistent grants support and use grant copy, but such a change is clearly more invasive and would need far more testing.
Persistent grants are hogging ballooned pages, which is equivalent to
memory only in case of the backend's domain memory being equal or
rather near to its max memory size.
So configuring the backend domain with enough spare area for ballooned
pages should make this problem much less serious.
Another problem in this area is the amount of maptrack frames configured
for a driver domain, which will limit the number of concurrent foreign
mappings of that domain.
So instead of having a blkback specific solution I'd rather have a
common callback for backends to release foreign mappings in order to
enable a global resource management.
Juergen
On Mon, 9 Dec 2019 10:39:02 +0100 Juergen <[email protected]> wrote:
>On 09.12.19 09:58, SeongJae Park wrote:
>> Each `blkif` has a free pages pool for the grant mapping. The size of
>> the pool starts from zero and be increased on demand while processing
>> the I/O requests. If current I/O requests handling is finished or 100
>> milliseconds has passed since last I/O requests handling, it checks and
>> shrinks the pool to not exceed the size limit, `max_buffer_pages`.
>>
>> Therefore, `blkfront` running guests can cause a memory pressure in the
>> `blkback` running guest by attaching a large number of block devices and
>> inducing I/O.
>
>I'm having problems to understand how a guest can attach a large number
>of block devices without those having been configured by the host admin
>before.
>
>If those devices have been configured, dom0 should be ready for that
>number of devices, e.g. by having enough spare memory area for ballooned
>pages.
As mentioned in the original message as below, administrators _can_ avoid this
problem, but finding the optimal configuration is hard, especially if the
number of the guests is large.
System administrators can avoid such problematic situations by limiting
the maximum number of devices each guest can attach. However, finding
the optimal limit is not so easy. Improper set of the limit can
results in the memory pressure or a resource underutilization.
Thanks,
SeongJae Park
>
>So either I'm missing something here or your reasoning for the need of
>the patch is wrong.
>
>
>Juergen
>
On Mon, 9 Dec 2019 11:15:22 +0100 "Jürgen Groß" <[email protected]> wrote:
>On 09.12.19 10:46, Durrant, Paul wrote:
>>> -----Original Message-----
>>> From: Jürgen Groß <[email protected]>
>>> Sent: 09 December 2019 09:39
>>> To: Park, Seongjae <[email protected]>; [email protected];
>>> [email protected]; [email protected]
>>> Cc: [email protected]; [email protected]; Durrant,
>>> Paul <[email protected]>; [email protected]; xen-
>>> [email protected]
>>> Subject: Re: [PATCH v3 0/1] xen/blkback: Squeeze page pools if a memory
>>> pressure
>>>
>>> On 09.12.19 09:58, SeongJae Park wrote:
>>>> Each `blkif` has a free pages pool for the grant mapping. The size of
>>>> the pool starts from zero and be increased on demand while processing
>>>> the I/O requests. If current I/O requests handling is finished or 100
>>>> milliseconds has passed since last I/O requests handling, it checks and
>>>> shrinks the pool to not exceed the size limit, `max_buffer_pages`.
>>>>
>>>> Therefore, `blkfront` running guests can cause a memory pressure in the
>>>> `blkback` running guest by attaching a large number of block devices and
>>>> inducing I/O.
>>>
>>> I'm having problems to understand how a guest can attach a large number
>>> of block devices without those having been configured by the host admin
>>> before.
>>>
>>> If those devices have been configured, dom0 should be ready for that
>>> number of devices, e.g. by having enough spare memory area for ballooned
>>> pages.
>>>
>>> So either I'm missing something here or your reasoning for the need of
>>> the patch is wrong.
>>>
>>
>> I think the underlying issue is that persistent grant support is hogging memory in the backends, thereby compromising scalability. IIUC this patch is essentially a band-aid to get back to the scalability that was possible before persistent grant support was added. Ultimately the right answer should be to get rid of persistent grants support and use grant copy, but such a change is clearly more invasive and would need far more testing.
>
>Persistent grants are hogging ballooned pages, which is equivalent to
>memory only in case of the backend's domain memory being equal or
>rather near to its max memory size.
>
>So configuring the backend domain with enough spare area for ballooned
>pages should make this problem much less serious.
>
>Another problem in this area is the amount of maptrack frames configured
>for a driver domain, which will limit the number of concurrent foreign
>mappings of that domain.
Right, similar problems from other backends are possible.
>
>So instead of having a blkback specific solution I'd rather have a
>common callback for backends to release foreign mappings in order to
>enable a global resource management.
This patch is also based on a common callback, namely the shrinker callback
system. As the shrinker callback is designed for the general memory pressure
handling, I thought this is a right one to use. Other backends having similar
problems can use this in their way.
Thanks,
SeongJae Park
>
>
>Juergen
>
On 09.12.19 11:52, SeongJae Park wrote:
> On Mon, 9 Dec 2019 11:15:22 +0100 "Jürgen Groß" <[email protected]> wrote:
>
>> On 09.12.19 10:46, Durrant, Paul wrote:
>>>> -----Original Message-----
>>>> From: Jürgen Groß <[email protected]>
>>>> Sent: 09 December 2019 09:39
>>>> To: Park, Seongjae <[email protected]>; [email protected];
>>>> [email protected]; [email protected]
>>>> Cc: [email protected]; [email protected]; Durrant,
>>>> Paul <[email protected]>; [email protected]; xen-
>>>> [email protected]
>>>> Subject: Re: [PATCH v3 0/1] xen/blkback: Squeeze page pools if a memory
>>>> pressure
>>>>
>>>> On 09.12.19 09:58, SeongJae Park wrote:
>>>>> Each `blkif` has a free pages pool for the grant mapping. The size of
>>>>> the pool starts from zero and be increased on demand while processing
>>>>> the I/O requests. If current I/O requests handling is finished or 100
>>>>> milliseconds has passed since last I/O requests handling, it checks and
>>>>> shrinks the pool to not exceed the size limit, `max_buffer_pages`.
>>>>>
>>>>> Therefore, `blkfront` running guests can cause a memory pressure in the
>>>>> `blkback` running guest by attaching a large number of block devices and
>>>>> inducing I/O.
>>>>
>>>> I'm having problems to understand how a guest can attach a large number
>>>> of block devices without those having been configured by the host admin
>>>> before.
>>>>
>>>> If those devices have been configured, dom0 should be ready for that
>>>> number of devices, e.g. by having enough spare memory area for ballooned
>>>> pages.
>>>>
>>>> So either I'm missing something here or your reasoning for the need of
>>>> the patch is wrong.
>>>>
>>>
>>> I think the underlying issue is that persistent grant support is hogging memory in the backends, thereby compromising scalability. IIUC this patch is essentially a band-aid to get back to the scalability that was possible before persistent grant support was added. Ultimately the right answer should be to get rid of persistent grants support and use grant copy, but such a change is clearly more invasive and would need far more testing.
>>
>> Persistent grants are hogging ballooned pages, which is equivalent to
>> memory only in case of the backend's domain memory being equal or
>> rather near to its max memory size.
>>
>> So configuring the backend domain with enough spare area for ballooned
>> pages should make this problem much less serious.
>>
>> Another problem in this area is the amount of maptrack frames configured
>> for a driver domain, which will limit the number of concurrent foreign
>> mappings of that domain.
>
> Right, similar problems from other backends are possible.
>
>>
>> So instead of having a blkback specific solution I'd rather have a
>> common callback for backends to release foreign mappings in order to
>> enable a global resource management.
>
> This patch is also based on a common callback, namely the shrinker callback
> system. As the shrinker callback is designed for the general memory pressure
> handling, I thought this is a right one to use. Other backends having similar
> problems can use this in their way.
But this is addressing memory shortage only and it is acting globally.
What I'd like to have in some (maybe distant) future is a way to control
resource usage per guest. Why would you want to throttle performance of
all guests instead of only the one causing the pain by hogging lots of
resources?
The new backend callback should (IMO) have a domid as parameter for
specifying which guest should be taken away resources (including the
possibility to select "any domain").
It might be reasonable to have your shrinker hook in e.g. xenbus for
calling the backend callbacks. And you could have another agent in the
grant driver reacting on shortage of possible grant mappings.
I don't expect you to implement all of that at once, but I think having
that idea in mind when addressing current issues would be nice. So as a
starting point you could move the shrinker hook to xenbus, add the
generic callback to struct xenbus_driver, populate that callback in
blkback and call it in the shrinker hook with "any domain". This would
enable a future extension to other backends and a dynamic resource
management in a natural way.
Juergen
On 09.12.19 11:23, SeongJae Park wrote:
> On Mon, 9 Dec 2019 10:39:02 +0100 Juergen <[email protected]> wrote:
>
>> On 09.12.19 09:58, SeongJae Park wrote:
>>> Each `blkif` has a free pages pool for the grant mapping. The size of
>>> the pool starts from zero and be increased on demand while processing
>>> the I/O requests. If current I/O requests handling is finished or 100
>>> milliseconds has passed since last I/O requests handling, it checks and
>>> shrinks the pool to not exceed the size limit, `max_buffer_pages`.
>>>
>>> Therefore, `blkfront` running guests can cause a memory pressure in the
>>> `blkback` running guest by attaching a large number of block devices and
>>> inducing I/O.
>>
>> I'm having problems to understand how a guest can attach a large number
>> of block devices without those having been configured by the host admin
>> before.
>>
>> If those devices have been configured, dom0 should be ready for that
>> number of devices, e.g. by having enough spare memory area for ballooned
>> pages.
>
> As mentioned in the original message as below, administrators _can_ avoid this
> problem, but finding the optimal configuration is hard, especially if the
> number of the guests is large.
>
> System administrators can avoid such problematic situations by limiting
> the maximum number of devices each guest can attach. However, finding
> the optimal limit is not so easy. Improper set of the limit can
> results in the memory pressure or a resource underutilization.
This sounds as if the admin would set a device limit. But it is the
other way round: The admin needs to configure each possible device
with all parameters (e.g. backing dom0 resource) for enabling the
frontend to use it.
Juergen
On Mon, 9 Dec 2019 12:08:10 +0100 "Jürgen Groß" <[email protected]> wrote:
>On 09.12.19 11:52, SeongJae Park wrote:
>> On Mon, 9 Dec 2019 11:15:22 +0100 "Jürgen Groß" <[email protected]> wrote:
>>
>>> On 09.12.19 10:46, Durrant, Paul wrote:
>>>>> -----Original Message-----
>>>>> From: Jürgen Groß <[email protected]>
>>>>> Sent: 09 December 2019 09:39
>>>>> To: Park, Seongjae <[email protected]>; [email protected];
>>>>> [email protected]; [email protected]
>>>>> Cc: [email protected]; [email protected]; Durrant,
>>>>> Paul <[email protected]>; [email protected]; xen-
>>>>> [email protected]
>>>>> Subject: Re: [PATCH v3 0/1] xen/blkback: Squeeze page pools if a memory
>>>>> pressure
>>>>>
>>>>> On 09.12.19 09:58, SeongJae Park wrote:
>>>>>> Each `blkif` has a free pages pool for the grant mapping. The size of
>>>>>> the pool starts from zero and be increased on demand while processing
>>>>>> the I/O requests. If current I/O requests handling is finished or 100
>>>>>> milliseconds has passed since last I/O requests handling, it checks and
>>>>>> shrinks the pool to not exceed the size limit, `max_buffer_pages`.
>>>>>>
>>>>>> Therefore, `blkfront` running guests can cause a memory pressure in the
>>>>>> `blkback` running guest by attaching a large number of block devices and
>>>>>> inducing I/O.
>>>>>
>>>>> I'm having problems to understand how a guest can attach a large number
>>>>> of block devices without those having been configured by the host admin
>>>>> before.
>>>>>
>>>>> If those devices have been configured, dom0 should be ready for that
>>>>> number of devices, e.g. by having enough spare memory area for ballooned
>>>>> pages.
>>>>>
>>>>> So either I'm missing something here or your reasoning for the need of
>>>>> the patch is wrong.
>>>>>
>>>>
>>>> I think the underlying issue is that persistent grant support is hogging memory in the backends, thereby compromising scalability. IIUC this patch is essentially a band-aid to get back to the scalability that was possible before persistent grant support was added. Ultimately the right answer should be to get rid of persistent grants support and use grant copy, but such a change is clearly more invasive and would need far more testing.
>>>
>>> Persistent grants are hogging ballooned pages, which is equivalent to
>>> memory only in case of the backend's domain memory being equal or
>>> rather near to its max memory size.
>>>
>>> So configuring the backend domain with enough spare area for ballooned
>>> pages should make this problem much less serious.
>>>
>>> Another problem in this area is the amount of maptrack frames configured
>>> for a driver domain, which will limit the number of concurrent foreign
>>> mappings of that domain.
>>
>> Right, similar problems from other backends are possible.
>>
>>>
>>> So instead of having a blkback specific solution I'd rather have a
>>> common callback for backends to release foreign mappings in order to
>>> enable a global resource management.
>>
>> This patch is also based on a common callback, namely the shrinker callback
>> system. As the shrinker callback is designed for the general memory pressure
>> handling, I thought this is a right one to use. Other backends having similar
>> problems can use this in their way.
>
> But this is addressing memory shortage only and it is acting globally.
>
> What I'd like to have in some (maybe distant) future is a way to control
> resource usage per guest. Why would you want to throttle performance of
> all guests instead of only the one causing the pain by hogging lots of
> resources?
Good point. I was also concerned about the performance fairness at first, but
settled in this ugly but simple solution mainly because my worst-case
performance test (detailed in 1st patch's commit msg) shows no visible
performance degradation, though it is a minimal test on my test environment.
Anyway, I agree with your future direction.
>
> The new backend callback should (IMO) have a domid as parameter for
> specifying which guest should be taken away resources (including the
> possibility to select "any domain").
>
> It might be reasonable to have your shrinker hook in e.g. xenbus for
> calling the backend callbacks. And you could have another agent in the
> grant driver reacting on shortage of possible grant mappings.
>
> I don't expect you to implement all of that at once, but I think having
> that idea in mind when addressing current issues would be nice. So as a
> starting point you could move the shrinker hook to xenbus, add the
> generic callback to struct xenbus_driver, populate that callback in
> blkback and call it in the shrinker hook with "any domain". This would
> enable a future extension to other backends and a dynamic resource
> management in a natural way.
Appreciate this kind and detailed advice. I will post the second version
applying your comments, soon.
Thanks,
SeongJae Park
>
>
>Juergen
>