Re: [PATCH mmotm] mm: alloc_large_system_hash check order

[Christoph Lameter <cl@linux.com>]

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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