linux-kernel - Re: [PATCH v5 6/7] zsmalloc: account the number of compacted pages

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Message-ID: <20150706134850.GB663@swordfish>
Date:	Mon, 6 Jul 2015 22:48:50 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 6/7] zsmalloc: account the number of compacted pages

On (07/06/15 22:22), Minchan Kim wrote:
> On Mon, Jul 06, 2015 at 09:17:49PM +0900, Sergey Senozhatsky wrote:
> > Compaction returns back to zram the number of migrated objects,
> > which is quite uninformative -- we have objects of different
> > sizes so user space cannot obtain any valuable data from that
> > number. Change compaction to operate in terms of pages and
> > return back to compaction issuer the number of pages that
> > were freed during compaction. So from now on `num_compacted'
> > column in zram<id>/mm_stat represents more meaningful value:
> > the number of freed (compacted) pages.
> 
> Fair enough.
>  
> The main reason I introduced num_migrated is to investigate
> the effieciency of compaction. ie, num_freed / num_migrated.
> However, I didn't put num_freed at that time so I can't get 
> my goal with only num_migrated.
>  
> We could put new knob num_compacted as well as num_migrated
> but I don't think we need it now. Zram's compaction would be
> much efficient compared to VM's compaction because we don't
> have any non-movable objects in zspages and we can account
> exact number of free slots.
> 
> So, I want to change name from num_migrated to num_compacted
> and maintain only it which is more useful for admin, you said.
> 
> It's not far since we introduced num_migrated so I don't think
> change name wouldn't be a big problem for userspace,(I hope).
> 

Hello,

Yes, num_migrated rename patch was on my table.
I was thinking about two variants:

struct zs_pool
  atomic_long_t		pages_allocated;
  unsigned long		num_compacted;

or

struct zs_pool
  atomic_long_t		pages_allocated;
  unsigned long		pages_compacted;

the latter looks even better I think. But I didn't come up with a
sane API name to get these stats-- zs_get_pages_compacted() is a bit
misleading. So I decided to keep num_compacted to make the patch set
smaller.

Hm, exporting a new `struct zs_pool_stat' to zram is probably a good
way to go.

	-ss
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