linux-kernel - Re: [PATCH 3/4] oom: oom-killer don't use permillage of system-ram internally

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Message-ID: <4DCD0845.10409@jp.fujitsu.com>
Date:	Fri, 13 May 2011 19:30:29 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	CAI Qian <caiqian@...hat.com>, avagin@...il.com,
	Andrey Vagin <avagin@...nvz.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Minchan Kim <minchan.kim@...il.com>,
	Hugh Dickins <hughd@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 3/4] oom: oom-killer don't use permillage of system-ram
 internally

(2011/05/11 8:40), David Rientjes wrote:
> On Tue, 10 May 2011, KOSAKI Motohiro wrote:
>
>> CAI Qian reported his kernel did hang-up if he ran fork intensive
>> workload and then invoke oom-killer.
>>
>> The problem is, Current oom calculation uses 0-1000 normalized value
>> (The unit is a permillage of system-ram). Its low precision make
>> a lot of same oom score. IOW, in his case, all processes have<1
>> oom score and internal integral calculation round it to 1. Thus
>> oom-killer kill ineligible process. This regression is caused by
>> commit a63d83f427 (oom: badness heuristic rewrite).
>>
>> The solution is, the internal calculation just use number of pages
>> instead of permillage of system-ram. And convert it to permillage
>> value at displaying time.
>>
>> This patch doesn't change any ABI (included  /proc/<pid>/oom_score_adj)
>> even though current logic has a lot of my dislike thing.
>>
>
> s/permillage/proportion/
>
> This is unacceptable, it does not allow users to tune oom_score_adj
> appropriately based on the scores exported by /proc/pid/oom_score to
> discount an amount of RAM from a thread's memory usage in systemwide,
> memory controller, cpuset, or mempolicy contexts.  This is only possible
> because the oom score is normalized.

You misunderstand the code. The patch doesn't change oom_score.
The patch change fs/proc too.

>
> What would be acceptable would be to increase the granularity of the score
> to 10000 or 100000 to differentiate between threads using 0.01% or 0.001%
> of RAM from each other, respectively.  The range of oom_score_adj would
> remain the same, however, and be multiplied by 10 or 100, respectively,
> when factored into the badness score baseline.  I don't believe userspace
> cares to differentiate between more than 0.1% of available memory.

Currently, SGI buy 16TB memory. 16TB x 0.1% = 1.6GB. I don't think your
fork bomb process use bigger than 1.6GB. Thus your patch is unacceptable.

So, please read the code again. or run it.

> The other issue that this patch addresses is the bonus given to root
> processes.  I agree that if a root process is using 4% of RAM that it
> should not be equal to all other threads using 1%.  I do believe that a
> root process using 60% of RAM should be equal priority to a thread using
> 57%, however.  Perhaps a compromise would be to give root processes a
> bonus of 1% for every 30% of RAM they consume?

I think you are talking about patch [4/4], right? patch [3/4] and [4/4]
are attacking another issue. big machine issue and root user issue.


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