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

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Date:   Thu, 22 Nov 2018 11:14:43 +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 (11/21/18 05:34), Minchan Kim wrote:
> > 
> > Just a thought,
> > 
> > I wonder if it will make sense (and if it will be possible) to writeback
> > idle _compressed_ objects. Right now we decompress, say, a perfectly
> > fine 400-byte compressed object to a PAGE_SIZE-d object and then push
> > it to the WB device. In this particular case it has a x10 bigger IO
> > pressure on flash. If we can write/read compressed object then we
> > will write and read 400-bytes, instead of PAGE_SIZE.
> 
> Although it has pros/cons, that's the my final goal although it would
> add much complicated stuffs. Sometime, we should have the feature.

So you plan to switch to "compressed objects" writeback?

> However, I want to go simple one first which is very valuable, too.

Flash wearout is a serious problem; maybe less of a problem on smart
phones, but much bigger on TVs and on other embedded devices that have
lifespans of 5+ years. With "writeback idle compressed" we can remove
the existing "writeback incompressible pages" and writeback only
"idle, compressed" pages.

The existing incompressible writeback is way too aggressive, and,
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.

It's a whole different story with idle, compressible pages writeback.

	-ss