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

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Date:	Mon, 6 Jul 2015 23:00:33 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	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 Mon, Jul 06, 2015 at 10:48:50PM +0900, Sergey Senozhatsky wrote:
> 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

Yeb.

> 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.

Agreed.

> 
> 	-ss

-- 
Kind regards,
Minchan Kim
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