linux-kernel - Re: [PATCH v6 0/3] mm: ZSWAP swap-out of mTHP folios

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Message-ID: <CAJD7tkbYufUeWWLh=WU1xgeVLMaoafPz+SHgtf+JtYWN+6PbHg@mail.gmail.com>
Date: Thu, 29 Aug 2024 16:54:53 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Nhat Pham <nphamcs@...il.com>
Cc: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, hannes@...xchg.org, chengming.zhou@...ux.dev, 
	usamaarif642@...il.com, ryan.roberts@....com, ying.huang@...el.com, 
	21cnbao@...il.com, akpm@...ux-foundation.org, nanhai.zou@...el.com, 
	wajdi.k.feghali@...el.com, vinodh.gopal@...el.com
Subject: Re: [PATCH v6 0/3] mm: ZSWAP swap-out of mTHP folios

On Thu, Aug 29, 2024 at 4:45 PM Nhat Pham <nphamcs@...il.com> wrote:
>
> On Thu, Aug 29, 2024 at 3:49 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> >
> > On Thu, Aug 29, 2024 at 2:27 PM Kanchana P Sridhar
> >
> > We are basically comparing zram with zswap in this case, and it's not
> > fair because, as you mentioned, the zswap compressed data is being
> > accounted for while the zram compressed data isn't. I am not really
> > sure how valuable these test results are. Even if we remove the cgroup
> > accounting from zswap, we won't see an improvement, we should expect a
> > similar performance to zram.
> >
> > I think the test results that are really valuable are case 1, where
> > zswap users are currently disabling CONFIG_THP_SWAP, and get to enable
> > it after this series.
>
> Ah, this is a good point.
>
> I think the point of comparing mTHP zswap v.s mTHP (SSD)swap is more
> of a sanity check. IOW, if mTHP swap outperforms mTHP zswap, then
> something is wrong (otherwise why would enable zswap - might as well
> just use swap, since SSD swap with mTHP >>> zswap with mTHP >>> zswap
> without mTHP).

Yeah, good point, but as you mention below..

>
> That said, I don't think this benchmark can show it anyway. The access
> pattern here is such that all the allocated memories are really cold,
> so swap to disk (or to zram, which does not account memory usage
> towards cgroup) is better by definition... And Kanchana does not seem
> to have access to setup with larger SSD swapfiles? :)

I think it's also the fact that the processes exit right after they
are done allocating the memory. So I think in the case of SSD, when we
stall waiting for IO some processes get to exit and free up memory, so
we need to do less swapping out in general because the processes are
more serialized. With zswap, all processes try to access memory at the
same time so the required amount of memory at any given point is
higher, leading to more thrashing.

I suggested keeping the memory allocated for a long time to even the
playing field, or we can make the processes keep looping and accessing
the memory (or part of it) for a while.

That being said, I think this may be a signal that the memory.high
throttling is not performing as expected in the zswap case. Not sure
tbh, but I don't think SSD swap should perform better than zswap in
that case.