linux-kernel - Re: [patch -mm] cpusets: add memory_slab

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Date:	Tue, 10 Mar 2009 16:50:56 -0400 (EDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Matt Mackall <mpm@...enic.com>,
	Paul Menage <menage@...gle.com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch -mm] cpusets: add memory_slab_hardwall flag

On Mon, 9 Mar 2009, David Rientjes wrote:

> On Mon, 9 Mar 2009, Christoph Lameter wrote:
>
> > > On large NUMA machines, it is currently possible for a very large
> > > percentage (if not all) of your slab allocations to come from memory that
> > > is distant from your application's set of allowable cpus.  Such
> > > allocations that are long-lived would benefit from having affinity to
> > > those processors.  Again, this is the typical use case for cpusets: to
> > > bind memory nodes to groups of cpus with affinity to it for the tasks
> > > attached to the cpuset.
> >
> > Can you show us a real workload that suffers from this issue?
> >
>
> We're more interested in the isolation characteristic, but that also
> benefits large NUMA machines by keeping nodes free of egregious amounts of
> slab allocated for remote cpus.

So no real workload just some isolation idea.

> > If you want to make sure that an allocation comes from a certain node then
> > specifying the node in kmalloc_node() will give you what you want.
> >
>
> That's essentially what the change does implicitly: it changes all
> kmalloc() calls to kmalloc_node() for current->mems_allowed.

Ok then you can use kmalloc_node?


> > The usage of kernel objects may not be cpuset specific. This is true for
> > other objects than inode and dentries well.
> >
>
> Yes, and that's why we require the cpuset hardwall on a configurable
> per-cpuset basis.  If a cpuset has set this option for its workload, then
> it is demanding object allocations from local memory.  Other cpusets that
> do not have memory_slab_hardwall set can still allocate from any cpu slab
> or partial slab, including those allocated for the hardwall cpuset.

You cannot hardwall something that is used in a shared way by processes in
multiple cpusets.
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