linux-kernel - Re: [PATCH] mm/zswap: Improve with alloc

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Date: Thu, 18 Jan 2024 09:06:43 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Nhat Pham <nphamcs@...il.com>, Ronald Monthero <debug.penguin32@...il.com>, sjenning@...hat.com, 
	ddstreet@...e.org, vitaly.wool@...sulko.com, akpm@...ux-foundation.org, 
	chrisl@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/zswap: Improve with alloc_workqueue() call

> > > On a different note, I wonder if it would help to perform synchronous
> > > reclaim here instead. With our current design, the zswap store failure
> > > (due to global limit hit) would leave the incoming page going to swap
> > > instead, creating an LRU inversion. Not sure if that's ideal.
> >
> > The global shrink path keeps reclaiming until zswap can accept again
> > (by default, that means reclaiming 10% of the total limit). I think
> > this is too expensive to be done synchronously.
>
> That thresholding code is a bit weird right now.
>
> It wakes the shrinker and rejects at the same time. We're guaranteed
> to see rejections, even if the shrinker has no trouble flushing some
> entries a split second later.
>
> It would make more sense to wake the shrinker at e.g. 95% full and
> have it run until 90%.
>
> But with that in place we also *should* do synchronous reclaim once we
> hit 100%. Just enough to make room for the store. This is important to
> catch the case where reclaim rate exceeds swapout rate. Rejecting and
> going to swap means the reclaimer will be throttled down to IO rate
> anyway, and the app latency isn't any worse. But this way we keep the
> pipeline alive, and keep swapping out the oldest zswap entries,
> instead of rejecting and swapping what would be the hottest ones.

I fully agree with the thresholding code being weird, and with waking
up the shrinker before the pool is full. What I don't understand is
how we can do synchronous reclaim when we hit 100% and still respect
the acceptance threshold :/

Are you proposing we change the semantics of the acceptance threshold
to begin with?