linux-kernel - Re: [PATCH 4/6] zram: support idle page writeback

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Date:   Thu, 22 Nov 2018 15:59:26 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/6] zram: support idle page writeback

On (11/22/18 15:31), Minchan Kim wrote:
> > 
> > I got what you mean now. Let's call it as "incompressible page wrieback"
> > to prevent confusing.
> > 
> > "incompressible page writeback" would be orthgonal feature. The goal is
> > "let's save memory at the cost of *latency*". If the page is swapped-in
> > soon, it's unfortunate. However, the design expects once it's swapped out,
> > it means it's non-workingset so soonish swappined-in would be rather not
> > many, theoritically compared to other workingset.
> > If's it's too frequent, it means system were heavily overcommitted.
> 
> Havid said, I agree it's not a good idea to enable incompressible page
> writeback with idle page writeback. If you don't oppose, I want to add
> new knob to "enable incompressible page writeback" so by default,
> although we enable CONFIG_ZRAM_WRITEBACK, incompressible page writeback
> is off until we enable the knob.
> It would make some regressison if someone have used the feature but
> I guess we are not too late.
> 
> What do you think?

Yes, totally works for me!

"IDLE writeback" is superior to "incompressible writeback".

"incompressible writeback" is completely unpredictable and
uncontrollable; it depens on data patterns and compression algorithms.
While "IDLE writeback" is predictable.

I even suspect, that, *ideally*, we can remove "incompressible
writeback". "IDLE pages" is a super set which also includes
"incompressible" pages. So, technically, we still can do
"incompressible writeback" from "IDLE writeback" path; but a much
more reasonable one, based on a page idling period.

I understand that you want to keep "direct incompressible writeback"
around. ZRAM is especially popular on devices which do suffer from
flash wearout, so I can see "incompressible writeback" path becoming
a dead code, long term.

	-ss