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

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Message-ID: <20181122061542.GA211816@google.com>
Date:   Thu, 22 Nov 2018 15:15:42 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/6] zram: support idle page writeback

On Thu, Nov 22, 2018 at 02:40:40PM +0900, Sergey Senozhatsky wrote:
> On (11/22/18 14:04), Minchan Kim wrote:
> > 
> > > additionally, it's too simple. It writes-back pages which can be
> > > swapped in immediately; which basically means that we do pointless
> > > PAGE_SIZE writes to a device which doesn't really like pointless
> > > writes.
> > 
> > This patchset aims for *IDLE page* writeback and you can define
> > what is IDLE page by yourself. It doesn't do pointless writeback.
> > > 
> > > It's a whole different story with idle, compressible pages writeback.
> > 
> > I don't understand your point.
> 
> Seems you misunderstood me. I'm not saying that IDLE writeback is bad.
> On the contrary, I think IDLE writeback is x100 better than writeback
> which we currently have.
> 
> The "pointless writeback" comment was about the existing writeback,
> when we WB pages which we couldn't compress. We can have a relative
> huge percentage of incompressible pages, and not all of them will end
> up being IDLE:
>  - we swap out page
>  - can't compress it
>  - writeback PAGE_SIZE
>  - swap it in two seconds later

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.