linux-kernel - Re: [RFC PATCH] mm/thp: Always allocate transparent hugepages on local node

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Date:	Tue, 25 Nov 2014 16:17:02 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	David Rientjes <rientjes@...gle.com>
Cc:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm/thp: Always allocate transparent hugepages on
 local node

On Mon, Nov 24, 2014 at 01:33:42PM -0800, David Rientjes wrote:
> On Mon, 24 Nov 2014, Kirill A. Shutemov wrote:
> 
> > > This make sure that we try to allocate hugepages from local node. If
> > > we can't we fallback to small page allocation based on
> > > mempolicy. This is based on the observation that allocating pages
> > > on local node is more beneficial that allocating hugepages on remote node.
> > 
> > Local node on allocation is not necessary local node for use.
> > If policy says to use a specific node[s], we should follow.
> > 
> 
> True, and the interaction between thp and mempolicies is fragile: if a 
> process has a MPOL_BIND mempolicy over a set of nodes, that does not 
> necessarily mean that we want to allocate thp remotely if it will always 
> be accessed remotely.  It's simple to benchmark and show that remote 
> access latency of a hugepage can exceed that of local pages.  MPOL_BIND 
> itself is a policy of exclusion, not inclusion, and it's difficult to 
> define when local pages and its cost of allocation is better than remote 
> thp.
> 
> For MPOL_BIND, if the local node is allowed then thp should be forced from 
> that node, if the local node is disallowed then allocate from any node in 
> the nodemask.  For MPOL_INTERLEAVE, I think we should only allocate thp 
> from the next node in order, otherwise fail the allocation and fallback to 
> small pages.  Is this what you meant as well?

Correct.

> > I think it makes sense to force local allocation if policy is interleave
> > or if current node is in preferred or bind set.
> >  
> 
> If local allocation were forced for MPOL_INTERLEAVE and all memory is 
> initially faulted by cpus on a single node, then the policy has 
> effectively become MPOL_DEFAULT, there's no interleave.

You're right. I don't have much experience with mempolicy code.

-- 
 Kirill A. Shutemov
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