Messages in this thread |  | | | Date | Fri, 1 May 2009 15:12:35 +0100 | | From | Mel Gorman <> | | Subject | Re: [PATCH mmotm] mm: alloc_large_system_hash check order |
| |
On Fri, May 01, 2009 at 03:00:15PM +0100, Mel Gorman wrote: > > <SNIP> > > > > 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).
I was foolish to believe the documentation. vmalloc() will be used by default on 64-bit NUMA, not just IA-64.
> 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(). >
I think this point still holds. On non-NUMA machine, we don't want to fall back to using vmalloc() just because the machine happened to have enough memory. It's really tricky to know for sure though - will there be enough performance benefits from having a bigger hash table to offset using base pages to back it? It's probably unknowable because it depends on the exact hardware and how the hash table is being used.
> [*] I speculate that on non-IA64 NUMA machines that we see different > performance for large filesystem benchmarks depending on whether we are > running on the boot-CPU node or not depending on whether hashdist= > is used or not.
This speculation is junk because using vmalloc() for hash tables is not specific to IA-64.
> <SNIP>
-- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab
|  |