Re: [PATCH v16.1 0/9] mm / virtio: Provide support for free page reporting

[David Hildenbrand]

On 23.01.20 11:20, Alexander Graf wrote:
> Hi Alex,
> 
> On 22.01.20 18:43, Alexander Duyck 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.
>>
>> 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, processed as many pages as were listed in
>> 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 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%
>>
>> 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.
>>
>> 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 overhead 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.
> 
> 
> I really like the approach overall. Voluntarily propagating free memory 
> from a guest to the host has been a sore point ever since KVM was 
> around. This solution looks like a very elegant way to do so.
> 
> The big piece I'm missing is the page cache. Linux will by default try 
> to keep the free list as small as it can in favor of page cache, so most 
> of the benefit of this patch set will be void in real world scenarios.

One approach is to move (parts of) the page cache from the guest to the
hypervisor - e.g., using emulated NVDIMM or virtio-pmem.

-- 
Thanks,

David / dhildenb