Messages in this thread | | | From | Eric Dumazet <> | Subject | Re: [RFC] NUMA futex hashing | Date | Tue, 8 Aug 2006 12:10:39 +0200 |
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On Tuesday 08 August 2006 11:57, Andi Kleen wrote: > Ravikiran G Thirumalai <kiran@scalex86.org> writes: > > Current futex hash scheme is not the best for NUMA. The futex hash > > table is an array of struct futex_hash_bucket, which is just a spinlock > > and a list_head -- this means multiple spinlocks on the same cacheline > > and on NUMA machines, on the same internode cacheline. If futexes of two > > unrelated threads running on two different nodes happen to hash onto > > adjacent hash buckets, or buckets on the same internode cacheline, then > > we have the internode cacheline bouncing between nodes. > > When I did some testing with a (arguably far too lock intensive) benchmark > on a bigger box I got most bouncing cycles not in the futex locks itself, > but in the down_read on the mm semaphore.
This is true, even with a normal application (not a biased benchmark) and using oprofile. mmap_sem is the killer.
We may have special case for PRIVATE futexes (they dont need to be chained in a global table, but a process private table)
POSIX thread api already can let the application tell glibc/kernel a mutex/futex ahe a process scope.
For this private futexes, I think we would not need to down_read(mmap_sem) at all. (only a/some lock/s protecting the process private table)
Eric
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