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

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Message-ID: <20181122054040.GE3441@jagdpanzerIV>
Date:   Thu, 22 Nov 2018 14:40:40 +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 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

// as example //

IDLE page writeback (this patch set) looks to me like a really
significant improvement. Especially if we can writeback compressed
objects and do, e.g., 300-bytes writes instead of PAGE_SIZE writes.

	-ss