linux-kernel - Re: [PATCH v2] vmscan: limit concurrent reclaimers in shrink

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Date:	Sun, 13 Dec 2009 23:29:45 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Minchan Kim <minchan.kim@...il.com>
CC:	lwoodman@...hat.com, akpm@...ux-foundation.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] vmscan: limit concurrent reclaimers in shrink_zone

On 12/13/2009 11:19 PM, Minchan Kim wrote:
> On Mon, Dec 14, 2009 at 1:09 PM, Rik van Riel<riel@...hat.com>  wrote:

>> A simpler solution may be to use sleep_on_interruptible, and
>> simply have the process continue into shrink_zone() if it
>> gets a signal.
>
> I thought it but I was not sure.
> Okay. If it is possible, It' more simple.
> Could you repost patch with that?

Sure, not a problem.

>          +The default value is 8.
>          +
>          +=============================================================
>
>
>      I like this. but why do you select default value as constant 8?
>      Do you have any reason?
>
>      I think it would be better to select the number proportional to NR_CPU.
>      ex) NR_CPU * 2 or something.
>
>      Otherwise looks good to me.
>
>
> Pessimistically, I assume that the pageout code spends maybe
> 10% of its time on locking (we have seen far, far worse than
> this with thousands of processes in the pageout code).  That
> means if we have more than 10 threads in the pageout code,
> we could end up spending more time on locking and less doing
> real work - slowing everybody down.
>
> I rounded it down to the closest power of 2 to come up with
> an arbitrary number that looked safe :)
> ===
>
> We discussed above.
> I want to add your desciption into changelog.

The thing is, I don't know if 8 is the best value for
the default, which is a reason I made it tunable in
the first place.

There are a lot of assumptions in my reasoning, and
they may be wrong.  I suspect that documenting something
wrong is probably worse than letting people wonder out
the default (and maybe finding a better one).

-- 
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