linux-kernel - Re: [PATCH -mm 2/2] cgroup: use multibuf for tasks file

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Message-Id: <20080919044828.00FA.KOSAKI.MOTOHIRO@jp.fujitsu.com>
Date:	Fri, 19 Sep 2008 04:52:12 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	kosaki.motohiro@...fujitsu.com, Li Zefan <lizf@...fujitsu.com>,
	Paul Menage <menage@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm 2/2] cgroup: use multibuf for tasks file

> Li Zefan wrote:
> > Lai Jiangshan wrote:
> >> Paul Menage wrote:
> >>> On Fri, Sep 12, 2008 at 4:55 AM, Lai Jiangshan <laijs@...fujitsu.com> wrote:
> >>>> when we open a really large cgroup for read, we may failed
> >>>> for kmalloc() is not reliable for allocate a big buffer.
> >>> This still depends on an answer to whether using plain vmalloc is too
> >>> much overhead.
> >>>
> >>> Balbir pointed out to me that most cgroups are likely to be pretty
> >>> small - so the solution of just doing a kmalloc() for 8K or less, and
> >>> a vmalloc() for more than 8K (which is >2000 threads) will avoid the
> >>> vmalloc overhead in almost all cases; the question is whether
> >>> eliminating the remaining overhead is worth the extra complexity.
> >>>
> >> I think open cgroup.tasks to read is not a critical path.
> >> so using plain vmalloc(even more overhead functions) is worth.
> >>
> > 
> > This patch does not only add runtime overhead, but also make code much more
> > complex, so the code is harder to read and harder to maintain, and object size
> > is increased, which means increased memory footprint.
> > 
> > And is there any reason not using plain vmalloc? Don't bloat the kernel without
> > good reasons IMO...
> > 
> 
> I said that vmalloc is worth.
> vmalloc was the fist choice of my opinion. ^_^

I agreed with Paul Menage's opinion because ..

 - plain vmalloc cause unnecessary overhead.
 - vmalloc sholdn't use for small allocation
   because virtual address space is valuable resource on 32bit machine.



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