linux-kernel - Re: + zram-support-compaction.patch added to -mm tree

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Date:	Mon, 9 Mar 2015 10:27:28 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	akpm@...ux-foundation.org, ddstreet@...e.org, gunho.lee@....com,
	iamjoonsoo.kim@....com, jmarchan@...hat.com, juno.choi@....com,
	mel@....ul.ie, ngupta@...are.org, semenzato@...gle.com,
	sergey.senozhatsky@...il.com, sjennings@...iantweb.net,
	mm-commits@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: + zram-support-compaction.patch added to -mm tree

On (03/09/15 10:05), Minchan Kim wrote:
> > well, to be fair, compaction is a zsmalloc internal. zram has nothing to do with
> > it.
> > 
> > but do we we even need this stat? it seems that
> > 
> >    mem_total_used (before compaction) - mem_total_user (after comapction)
> > 
> > will give user an idea on how much memory was compacted.
> 
> It's not enough. What I want to know is compaction efficiency per client of
> zsmalloc(ie, zram). 
>

so what a typical user can do with this information? isn't it an entirely
debug info that makes some hidden sense only to developers?

if you insist on exporting this as a zram stat for everyone how obout
starting to move away from per-stat RO sysfs attrs. it seems that we have
uncomfortably a lot of sysfs attrs, and that doesn't make life easier in
user space. for example, block devices have /sys/block/.../stat file:

/sys/block/sda$ cat stat
   45931       59  2075686   289906    55768     9229  1967800   318033        0   193583   607806

and there are no num_reads, num_writes, num_failed_reads, num_failed_writes,
etc., etc. per-stat sysfs attrs force user-space to do lots of syscalls:
open(), read(), close() with error control on every step; for every stat.

so how about introducing zram<id>/malloc_stats (or any similar name) and
provide compaction and all future allocator related stats there
(via s*printf("%d %d %d", ....)) ?

	-ss

> IOW, (how many of freed pages / how many of objects) per
> zs_compact.
> 
> > 
> > 	-ss
> > 
> > > That's why it is on debugfs. If we add the stat into zsmalloc, we should turn on debugfs
> > > and CONFIG_ZSMALLOC_STAT to see *a* stat. Even, CONFIG_ZSMALLOC_STAT will add
> > > unncessary overheads to account another stats fo zsmalloc internals.
> > > 
> > > As well, if we add auto-compacion like stuff in zsmalloc(ie, it will trigger
> > > by itself if fragmention is over to predefined theshold), the stat will
> > > accumulate stat while someone want to see snapshot compaction effiecieny
> > > of the moment.
> > > 
> > > So, I want to keep it in zram now.
> > > 
> > > -- 
> > > Kind regards,
> > > Minchan Kim
> > > 
> 
> -- 
> Kind regards,
> Minchan Kim
> 
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