Re: [PATCH v14 0/6] mm / virtio: Provide support for unused page reporting

[David Hildenbrand]

On 27.11.19 18:36, Alexander Duyck wrote:
> On Wed, 2019-11-27 at 11:01 +0100, David Hildenbrand wrote:
>> On 26.11.19 17:45, Alexander Duyck wrote:
>>> On Tue, 2019-11-26 at 13:20 +0100, David Hildenbrand wrote:
>>>> On 19.11.19 22:46, Alexander Duyck wrote:
> 
> <snip>
> 
>>>>> 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 power saving features disabled
>>>>> by setting the /dev/cpu_dma_latency value to 10ms.
>>>>>
>>>>> Test                page_fault1 (THP)     page_fault2
>>>>> Name         tasks  Process Iter  STDEV  Process Iter  STDEV
>>>>> Baseline         1    1203934.75  0.04%     379940.75  0.11%
>>>>>                 16    8828217.00  0.85%    3178653.00  1.28%
>>>>>
>>>>> Patches applied  1    1207961.25  0.10%     380852.25  0.25%
>>>>>                 16    8862373.00  0.98%    3246397.25  0.68%
>>>>>
>>>>> Patches enabled  1    1207758.75  0.17%     373079.25  0.60%
>>>>>  MADV disabled  16    8870373.75  0.29%    3204989.75  1.08%
>>>>>
>>>>> Patches enabled  1    1261183.75  0.39%     373201.50  0.50%
>>>>>                 16    8371359.75  0.65%    3233665.50  0.84%
>>>>>
>>>>> Patches enabled  1    1090201.50  0.25%     376967.25  0.29%
>>>>>  page shuffle   16    8108719.75  0.58%    3218450.25  1.07%
>>>>>
>>>>> The results above are for a baseline with a linux-next-20191115 kernel,
>>>>> that kernel with this patch set applied but page reporting disabled in
>>>>> virtio-balloon, patches applied but the madvise disabled by direct
>>>>> assigning a device, the patches applied and page reporting fully
>>>>> enabled, and the patches enabled with page shuffling enabled.  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.
>>>>>
>>>>> Most of the overhead seen 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.
>>>>
>>>> MADV_FREE would be interesting.
>>>
>>> I can probably code something up. However that is going to push a bunch of
>>> complexity into the QEMU code and doesn't really mean much to the kernel
>>> code. I can probably add it as another QEMU patch to the set since it is
>>> just a matter of having a function similar to ram_block_discard_range that
>>> uses MADV_FREE instead of MADV_DONTNEED.
>>
>> Yes, addon patch makes perfect sense. The nice thing about MADV_FREE is
>> that you only take back pages from a process when really under memory
>> pressure (before going to SWAP). You will still get a pagefault on the
>> next access (to identify that the page is still in use after all), but
>> don't have to fault in a fresh page.
> 
> So I got things running with a proof of concept using MADV_FREE.
> Apparently another roadblock I hadn't realized is that you have to have
> the right version of glibc for MADV_FREE to be present.
> 
> Anyway with MADV_FREE the numbers actually look pretty close to the
> numbers with the madvise disabled. Apparently the page fault overhead
> isn't all that significant. When I push the next patch set I will include
> the actual numbers, but even with shuffling enabled the results were in
> the 8.7 to 8.8 million iteration range.
> 

Cool, thanks for evaluating!

-- 
Thanks,

David / dhildenb