Received-SPF: pass (google.com: domain of linux-kernel+bounces-30430-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Date: Thu, 18 Jan 2024 12:39:27 -0500
From: Johannes Weiner <hannes@cmpxchg.org>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Nhat Pham <nphamcs@gmail.com>,
	Ronald Monthero <debug.penguin32@gmail.com>, sjenning@redhat.com,
	ddstreet@ieee.org, vitaly.wool@konsulko.com,
	akpm@linux-foundation.org, chrisl@kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/zswap: Improve with alloc_workqueue() call
Message-ID: <20240118173927.GL939255@cmpxchg.org>
References: <CAKEwX=NLe-N6dLvOVErPSL3Vfw6wqHgcUBQoNRLeWkN6chdvLQ@mail.gmail.com>
 <20240116133145.12454-1-debug.penguin32@gmail.com>
 <CAKEwX=PjraCg_NjP4Tnkbv8uqnVw8yJGh-mbuZC02Gp6HMcDBw@mail.gmail.com>
 <CAJD7tkb_uC_K7+C3GjVqg1rDRCmUkbHcEw950CkUHG66yokbcg@mail.gmail.com>
 <20240118161601.GJ939255@cmpxchg.org>
 <CAJD7tkb+NCocjpjvM08Nv2fPMDqm4t-YH05TJckz8m1AYFNWUg@mail.gmail.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAJD7tkb+NCocjpjvM08Nv2fPMDqm4t-YH05TJckz8m1AYFNWUg@mail.gmail.com>

On Thu, Jan 18, 2024 at 09:06:43AM -0800, Yosry Ahmed wrote:
> > > > 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?

I kind of am. It's worth looking at the history of this knob.

It was added in 2020 by 45190f01dd402112d3d22c0ddc4152994f9e1e55, and
from the changelogs and the code in this patch I do not understand how
this was supposed to work.

It also *didn't* work for very basic real world applications. See
Domenico's follow-up (e0228d590beb0d0af345c58a282f01afac5c57f3), which
effectively reverted it to get halfway reasonable behavior.

If there are no good usecases for this knob, then I think it makes
sense to phase it out again.