linux-kernel - Re: [PATCH] remove memory.limit v.s. memsw.limit comparison.

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Message-ID: <0921392c77890fc84fa69653ae4f31d9.squirrel@webmail-b.css.fujitsu.com>
Date:	Fri, 5 Jun 2009 00:45:03 +0900 (JST)
From:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH] remove memory.limit v.s. memsw.limit comparison.

Balbir Singh wrote:
> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2009-06-04
> 14:10:43]:
>
>> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>>
>> Removes memory.limit < memsw.limit at setting limit check completely.
>>
>> The limitation "memory.limit <= memsw.limit" was added just because
>> it seems sane ...if memory.limit > memsw.limit, only memsw.limit works.
>>
>> But To implement this limitation, we needed to use private mutex and
>> make
>> the code a bit complated.
>> As Nishimura pointed out, in real world, there are people who only want
>> to use memsw.limit.
>>
>> Then, this patch removes the check. user-land library or middleware can
>> check
>> this in userland easily if this really concerns.
>>
>> And this is a good change to charge-and-reclaim.
>>
>> Now, memory.limit is always checked before memsw.limit
>> and it may do swap-out. But, if memory.limit == memsw.limit, swap-out is
>> finally no help and hits memsw.limit again. So, let's allow the
>> condition
>> memory.limit > memsw.limit. Then we can skip unnecesary swap-out.
>>
>> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>> ---
>
> There is one other option, we could set memory.limit_in_bytes ==
> memory.memsw.limit_in_bytes provided it is set to LONG_LONG_MAX. I am
> not convinced that we should allow memsw.limit_in_bytes to be less
> that limit_in_bytes, it will create confusion and the API is already
> exposed.
>
Ahhhh, I get your point.
 if memory.memsw.limit_in_bytes < memory.limit_in_bytes, no swap will
 be used bacause currnet try_to_free_pages() for memcg skips swap-out.
 Then, only global-LRU will use swap.
 This behavior is not easy to understand.

Sorry, I don't push this patch as this is. But adding documentation about
"What happens when you set memory.limit == memsw.limit" will be necessary.

...maybe give all jobs to user-land and keep the kernel as it is now
is a good choice.

BTW, I'd like to avoid useless swap-out in memory.limit == memsw.limit case.
If someone has good idea, please :(

Thanks,
-Kame


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