linux-kernel - Re: [PATCHSET] block, mempool, percpu: implement percpu mempool and fix blkcg percpu alloc deadlock

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Message-Id: <20111227132501.ad7f895f.akpm@linux-foundation.org>
Date:	Tue, 27 Dec 2011 13:25:01 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Tejun Heo <tj@...nel.org>, avi@...hat.com, nate@...nel.net,
	cl@...ux-foundation.org, oleg@...hat.com, axboe@...nel.dk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHSET] block, mempool, percpu: implement percpu mempool and
 fix blkcg percpu alloc deadlock

On Fri, 23 Dec 2011 09:58:56 -0500
Vivek Goyal <vgoyal@...hat.com> wrote:

> > Why do the allocation during I/O?  Can't it be done in the hotplug handler?
> > 
> 
> Even if we can do it in hotplug handler it will be very wasteful of
> memory. So if there are 100 IO cgroups in the system, upon every block
> device hotplug, we will allocate per cpu memory for all the 100 cgroups,
> irrespective of the fact whether they are doing IO to the device or not.
> 
> Now expand this to a system with 100 cgroups and 100 Luns. 10000
> allocations for no reason. (Even if we do it for cgroups needing stats,
> does not help much). Current scheme allocates memory for the group
> only if a sepcific cgroup is doing IO to a specific block device. 

umm, we've already declared that it is OK to completely waste this
memory for the users (probably a majority) who will not be using
these stats.

Also, has this stuff been tested at that scale?  I fear to think what
10000 allocations will do to fragmetnation of the vmalloc() arena.

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