netdev - Re: [PATCH mmotm] mm: alloc_large_system

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Message-ID: <alpine.DEB.1.10.0905010958090.18324@qirst.com>
Date:	Fri, 1 May 2009 09:59:35 -0400 (EDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Mel Gorman <mel@....ul.ie>
cc:	Hugh Dickins <hugh@...itas.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH mmotm] mm: alloc_large_system_hash check order

On Fri, 1 May 2009, Mel Gorman wrote:

> > Andrew noticed another oddity: that if it goes the hashdist __vmalloc()
> > way, it won't be limited by MAX_ORDER.  Makes one wonder whether it
> > ought to fall back to __vmalloc() if the alloc_pages_exact() fails.
>
> I don't believe so. __vmalloc() is only used when hashdist= is used or on IA-64
> (according to the documentation). It is used in the case that the caller is
> willing to deal with the vmalloc() overhead (e.g. using base page PTEs) in
> exchange for the pages being interleaved on different nodes so that access
> to the hash table has average performance[*]
>
> If we automatically fell back to vmalloc(), I bet 2c we'd eventually get
> a mysterious performance regression report for a workload that depended on
> the hash tables performance but that there was enough memory for the hash
> table to be allocated with vmalloc() instead of alloc_pages_exact().

Can we fall back to a huge page mapped vmalloc? Like what the vmemmap code
does? Then we also would not have MAX_ORDER limitations.
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