On Tue, 25 Jan 2011, Anton Blanchard wrote: > From: David Gibson <dwg@xxxxxxxxxxx> > > At present, the page fault path for hugepages is serialized by a > single mutex. This is used to avoid spurious out-of-memory conditions > when the hugepage pool is fully utilized (two processes or threads can > race to instantiate the same mapping with the last hugepage from the > pool, the race loser returning VM_FAULT_OOM). This problem is > specific to hugepages, because it is normal to want to use every > single hugepage in the system - with normal pages we simply assume > there will always be a few spare pages which can be used temporarily > until the race is resolved. > > Unfortunately this serialization also means that clearing of hugepages > cannot be parallelized across multiple CPUs, which can lead to very > long process startup times when using large numbers of hugepages. > > This patch improves the situation by replacing the single mutex with a > table of mutexes, selected based on a hash of the address_space and > file offset being faulted (or mm and virtual address for MAP_PRIVATE > mappings). > > > From: Anton Blanchard <anton@xxxxxxxxx> > > Forward ported and made a few changes: > > - Use the Jenkins hash to scatter the hash, better than using just the > low bits. > > - Always round num_fault_mutexes to a power of two to avoid an expensive > modulus in the hash calculation. > > I also tested this patch on a 64 thread POWER6 box using a simple parallel > fault testcase: > > http://ozlabs.org/~anton/junkcode/parallel_fault.c > > Command line options: > > parallel_fault <nr_threads> <size in kB> <skip in kB> > > First the time taken to fault 48GB of 16MB hugepages: > # time hugectl --heap ./parallel_fault 1 50331648 16384 > 11.1 seconds > > Now the same test with 64 concurrent threads: > # time hugectl --heap ./parallel_fault 64 50331648 16384 > 8.8 seconds > > Hardly any speedup. Finally the 64 concurrent threads test with this patch > applied: > # time hugectl --heap ./parallel_fault 64 50331648 16384 > 0.7 seconds > > We go from 8.8 seconds to 0.7 seconds, an improvement of 12.6x. > > Signed-off-by: David Gibson <dwg@xxxxxxxxxxx> > Signed-off-by: Anton Blanchard <anton@xxxxxxxxx> Reviewed-by: Eric B Munson <emunson@xxxxxxxxx>
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