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Subject: Re: [PATCH v16 0/9] mm / virtio: Provide support for free page
 reporting
To:     Alexander Duyck <alexander.duyck@gmail.com>, kvm@vger.kernel.org,
        mst@redhat.com, linux-kernel@vger.kernel.org, willy@infradead.org,
        mhocko@kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org,
        mgorman@techsingularity.net, vbabka@suse.cz
Cc:     yang.zhang.wz@gmail.com, konrad.wilk@oracle.com, david@redhat.com,
        pagupta@redhat.com, riel@surriel.com, lcapitulino@redhat.com,
        dave.hansen@intel.com, wei.w.wang@intel.com, aarcange@redhat.com,
        pbonzini@redhat.com, dan.j.williams@intel.com,
        alexander.h.duyck@linux.intel.com, osalvador@suse.de
References: <20200103210509.29237.18426.stgit@localhost.localdomain>
From:   Nitesh Narayan Lal <nitesh@redhat.com>
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Message-ID: <aebf72d6-3383-fcc5-7cea-efb930e4e245@redhat.com>
Date:   Wed, 8 Jan 2020 02:57:31 -0500
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On 1/3/20 4:16 PM, Alexander Duyck wrote:
> This series provides an asynchronous means of reporting free guest page=
s
> to a hypervisor so that the memory associated with those pages can be
> dropped and reused by other processes and/or guests on the host. Using
> this it is possible to avoid unnecessary I/O to disk and greatly improv=
e
> performance in the case of memory overcommit on the host.
>
> When enabled we will be performing a scan of free memory every 2 second=
s
> while pages of sufficiently high order are being freed. In each pass at=

> least one sixteenth of each free list will be reported. By doing this w=
e
> avoid racing against other threads that may be causing a high amount of=

> memory churn.
>
> The lowest page order currently scanned when reporting pages is
> pageblock_order so that this feature will not interfere with the use of=

> Transparent Huge Pages in the case of virtualization.
>
> Currently this is only in use by virtio-balloon however there is the ho=
pe
> that at some point in the future other hypervisors might be able to mak=
e
> use of it. In the virtio-balloon/QEMU implementation the hypervisor is
> currently using MADV_DONTNEED to indicate to the host kernel that the p=
age
> is currently free. It will be zeroed and faulted back into the guest th=
e
> next time the page is accessed.
>
> To track if a page is reported or not the Uptodate flag was repurposed =
and
> used as a Reported flag for Buddy pages. We walk though the free list
> isolating pages and adding them to the scatterlist until we either
> encounter the end of the list, processed as many pages as were listed i=
n
> nr_free prior to us starting, or have filled the scatterlist with pages=
 to
> be reported. If we fill the scatterlist before we reach the end of the
> list we rotate the list so that the first unreported page we encounter =
is
> moved to the head of the list as that is where we will resume after we
> have freed the reported pages back into the tail of the list.
>
> Below are the results from various benchmarks. I primarily focused on t=
wo
> tests. The first is the will-it-scale/page_fault2 test, and the other i=
s
> a modified version of will-it-scale/page_fault1 that was enabled to use=

> THP. I did this as it allows for better visibility into different parts=

> of the memory subsystem. The guest is running with 32G for RAM on one
> node of a E5-2630 v3. The host has had some features such as CPU turbo
> disabled in the BIOS.
>
> Test                   page_fault1 (THP)    page_fault2
> Name            tasks  Process Iter  STDEV  Process Iter  STDEV
> Baseline            1    1012402.50  0.14%     361855.25  0.81%
>                    16    8827457.25  0.09%    3282347.00  0.34%
>
> Patches Applied     1    1007897.00  0.23%     361887.00  0.26%
>                    16    8784741.75  0.39%    3240669.25  0.48%
>
> Patches Enabled     1    1010227.50  0.39%     359749.25  0.56%
>                    16    8756219.00  0.24%    3226608.75  0.97%
>
> Patches Enabled     1    1050982.00  4.26%     357966.25  0.14%
>  page shuffle      16    8672601.25  0.49%    3223177.75  0.40%
>
> Patches Enabled     1    1003238.00  0.22%     360211.00  0.22%
>  shuffle w/ RFC    16    8767010.50  0.32%    3199874.00  0.71%

Just to be sure that I understand your test setup correctly:
- You have a 32GB guest with a single node affined to a single node of yo=
ur host
(E5-2630).
- You have THP in both host and the guest enabled and set to 'madvise'.
- On top of the default x86_64 config and other virtio config options you=
 have
CONFIG_SLAB_FREELIST_RANDOM and CONFIG_SHUFFLE_PAGE_ALLOCATOR enabled for=
 the
third observation (Patches Enabled page shuffle).
did I miss anything?

Can you also remind me of the reason you have skipped recording the numbe=
r of
threads count reported as part of page_fault tests? Was it because you we=
re
observing different values with every fresh boot?


> The results above are for a baseline with a linux-next-20191219 kernel,=

> that kernel with this patch set applied but page reporting disabled in
> virtio-balloon, the patches applied and page reporting fully enabled, t=
he
> patches enabled with page shuffling enabled, and the patches applied wi=
th
> page shuffling enabled and an RFC patch that makes used of MADV_FREE in=

> QEMU. These results include the deviation seen between the average valu=
e
> reported here versus the high and/or low value. I observed that during =
the
> test memory usage for the first three tests never dropped whereas with =
the
> patches fully enabled the VM would drop to using only a few GB of the
> host's memory when switching from memhog to page fault tests.

Do you mean that in the later case you run the page fault tests after mem=
hog?
If so how much memory do you pass to memhog?

--=20
Nitesh