Re: [PATCH v2 0/5] variable-order, large folios for anonymous memory

[Yin, Fengwei]

On 7/4/2023 10:18 AM, Yu Zhao wrote:
> On Mon, Jul 3, 2023 at 7:53 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>>
>> Hi All,
>>
>> This is v2 of a series to implement variable order, large folios for anonymous
>> memory. The objective of this is to improve performance by allocating larger
>> chunks of memory during anonymous page faults. See [1] for background.
> 
> Thanks for the quick response!
> 
>> I've significantly reworked and simplified the patch set based on comments from
>> Yu Zhao (thanks for all your feedback!). I've also renamed the feature to
>> VARIABLE_THP, on Yu's advice.
>>
>> The last patch is for arm64 to explicitly override the default
>> arch_wants_pte_order() and is intended as an example. If this series is accepted
>> I suggest taking the first 4 patches through the mm tree and the arm64 change
>> could be handled through the arm64 tree separately. Neither has any build
>> dependency on the other.
>>
>> The one area where I haven't followed Yu's advice is in the determination of the
>> size of folio to use. It was suggested that I have a single preferred large
>> order, and if it doesn't fit in the VMA (due to exceeding VMA bounds, or there
>> being existing overlapping populated PTEs, etc) then fallback immediately to
>> order-0. It turned out that this approach caused a performance regression in the
>> Speedometer benchmark.
> 
> I suppose it's regression against the v1, not the unpatched kernel.
>From the performance data Ryan shared, it's against unpatched kernel:

Speedometer 2.0:

| kernel                         |   runs_per_min |
|:-------------------------------|---------------:|
| baseline-4k                    |           0.0% |
| anonfolio-lkml-v1              |           0.7% |
| anonfolio-lkml-v2-simple-order |          -0.9% |
| anonfolio-lkml-v2              |           0.5% |


What if we use 32K or 16K instead of 64K as default anonymous folio size? I suspect
this app may have 32K or 16K anon folio as sweet spot.


Regards
Yin, Fengwei

> 
>> With my v1 patch, there were significant quantities of
>> memory which could not be placed in the 64K bucket and were instead being
>> allocated for the 32K and 16K buckets. With the proposed simplification, that
>> memory ended up using the 4K bucket, so page faults increased by 2.75x compared
>> to the v1 patch (although due to the 64K bucket, this number is still a bit
>> lower than the baseline). So instead, I continue to calculate a folio order that
>> is somewhere between the preferred order and 0. (See below for more details).
> 
> I suppose the benchmark wasn't running under memory pressure, which is
> uncommon for client devices. It could be easier the other way around:
> using 32/16KB shows regression whereas order-0 shows better
> performance under memory pressure.
> 
> I'm not sure we should use v1 as the baseline. Unpatched kernel sounds
> more reasonable at this point. If 32/16KB is proven to be better in
> most scenarios including under memory pressure, we can reintroduce
> that policy. I highly doubt this is the case: we tried 16KB base page
> size on client devices, and overall, the regressions outweighs the
> benefits.
> 
>> The patches are based on top of v6.4 plus Matthew Wilcox's set_ptes() series
>> [2], which is a hard dependency. I have a branch at [3].
> 
> It's not clear to me why [2] is a hard dependency.
> 
> It seems to me we are getting close and I was hoping we could get into
> mm-unstable soon without depending on other series...
>