Re: [PATCH v17 0/9] mm / virtio: Provide support for free page reporting

[Andrew Morton]

On Tue, 11 Feb 2020 14:45:51 -0800 Alexander Duyck <alexander.duyck@xxxxxxxxx> wrote:

> This series provides an asynchronous means of reporting free guest pages
> 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 improve
> performance in the case of memory overcommit on the host.

"greatly improve" sounds nice.

> When enabled we will be performing a scan of free memory every 2 seconds
> 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 we
> 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 hope
> that at some point in the future other hypervisors might be able to make
> use of it. In the virtio-balloon/QEMU implementation the hypervisor is
> currently using MADV_DONTNEED to indicate to the host kernel that the page
> is currently free. It will be zeroed and faulted back into the guest the
> 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 or have processed at least one sixteenth of
> the pages that were listed in nr_free prior to us starting. 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 two
> tests. The first is the will-it-scale/page_fault2 test, and the other is
> 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%

But these differences seem really small - around 1%?  I think we're
just showing not much harm was caused?

> 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, the
> patches enabled with page shuffling enabled, and the patches applied with
> page shuffling enabled and an RFC patch that makes used of MADV_FREE in
> QEMU. These results include the deviation seen between the average value
> 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.

And this is the "great improvement", yes?

Is it possible to measure the end-user-visible benefits of this?

> Any of the overhead visible with this patch set enabled seems due to page
> faults caused by accessing the reported pages and the host zeroing the page
> before giving it back to the guest. This overhead is much more visible when
> using THP than with standard 4K pages. In addition page shuffling seemed to
> increase the amount of faults generated due to an increase in memory churn.
> The overehad is reduced when using MADV_FREE as we can avoid the extra
> zeroing of the pages when they are reintroduced to the host, as can be seen
> when the RFC is applied with shuffling enabled.
> 
> The overall guest size is kept fairly small to only a few GB while the test
> is running. If the host memory were oversubscribed this patch set should
> result in a performance improvement as swapping memory in the host can be
> avoided.

"should result".  Can we firm this up a lot?