linux-kernel - Re: [PATCH v5 0/3] mm: ZSWAP swap-out of mTHP folios

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Message-ID: <CAJD7tkZnT2_JD90xAt09Uk6Lsc6C8t0V5dpUOKLLknw75yUqkA@mail.gmail.com>
Date: Thu, 29 Aug 2024 16:38:37 -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, 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 v5 0/3] mm: ZSWAP swap-out of mTHP folios

On Thu, Aug 29, 2024 at 4:33 PM Nhat Pham <nphamcs@...il.com> wrote:
>
> On Wed, Aug 28, 2024 at 3:38 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> >
> > On Wed, Aug 28, 2024 at 2:35 AM Kanchana P Sridhar
> > <kanchana.p.sridhar@...el.com> wrote:
> > Are you saying that in the "Before" data we end up skipping zswap
> > completely because of using mTHPs?
> >
> > Does it make more sense to turn CONFIG_THP_SWAP in the "Before" data
> > to force the mTHPs to be split and for the data to be stored in zswap?
> > This would be a more fair Before/After comparison where the memory
> > goes to zswap in both cases, but "Before" has to be split because of
> > zswap's lack of support for mTHP. I assume most setups relying on
> > zswap will be turning CONFIG_THP_SWAP off today anyway, but maybe not.
> > Nhat, is this something you can share?
>
> I think we're enabling it, but we're a zswap heavy shop + THP
> allocation is not suuuper reliable until recently with Johannes'
> latest (and upcoming) work, so I don't have much data to share :)

Interesting. If CONFIG_THP_SWAP is enabled this basically means your
zswap utilization keeps going down as your THP utilization goes up. So
the swapin cost would go higher. How do you deal with that?