linux-kernel - Re: [PATCH 2/2] zram: idle memory tracking

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Date:   Tue, 27 Mar 2018 10:47:34 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Minchan Kim <minchan@...nel.org>,
        ndrew Morton <akpm@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH 2/2] zram: idle memory tracking

On (03/27/18 10:13), Sergey Senozhatsky wrote:
> On (03/27/18 10:03), Minchan Kim wrote:
> > On Tue, Mar 27, 2018 at 09:49:11AM +0900, Sergey Senozhatsky wrote:
> > > On (03/26/18 15:49), Minchan Kim wrote:
> > > > zRam as swap is useful for small memory device. However, swap means
> > > > those pages on zram are mostly cold pages due to VM's LRU algorithm.
> > > > Especially, once init data for application are touched for launching,
> > > > they tend to be not accessed any more and finally swapped out.
> > > > zRAM can store such cold pages as compressed form but it's pointless
> > > > to keep in memory. Better idea is app developers free them directly
> > > > rather than remaining them on heap.
> > > > 
> > > > This patch tell us last accesss time of each block of zram via
> > > > "cat /sys/kernel/debug/zram/zram0/access_time".
> > > > 
> > > > The output is as follows,
> > > > 
> > > > 276 1250
> > > > 277 1800
> > > > ..  ..
> > > > ..  ..
> > > 
> > > So can we just use CONFIG_IDLE_PAGE_TRACKING + CONFIG_PAGE_OWNER?
> > 
> > The goal is not mapped memory tracking but one swapped out.
> > Can we do it by above two combination?
> 
> Ah, you are right. page_idle tracks only pages which are on LRU lists.

Can a universal (not specific to zram) idle swapped out page tracking
be of interest to MM people? With the exactly same motivation. Some
swaps use nfs, for instance, so idle swap pages tracking can be helpful
not only in embedded domain.

	-ss