Re: [rfc] [patch 1/2 ] Process private hash tables for private futexes

Eric Dumazet
2009-03-21T09:08:21Z

Ravikiran G Thirumalai a écrit :
> Patch to have a process private hash table for 'PRIVATE' futexes.
>
> On large core count systems running multiple threaded processes causes
> false sharing on the global futex hash table. The global futex hash
> table is an array of struct futex_hash_bucket which is defined as:
>
> struct futex_hash_bucket {
> spinlock_t lock;
> struct plist_head chain;
> };
>
> static struct futex_hash_bucket futex_queues[1<<FUTEX_HASHBITS];
>
> Needless to say this will cause multiple spinlocks to reside on the
> same cacheline which is very bad when multiple un-related process
> hash onto adjacent hash buckets. The probability of unrelated futexes
> ending on adjacent hash buckets increase with the number of cores in the
> system (more cores available translates to more processes/more threads
> being run on a system). The effects of false sharing are tangible on
> machines with more than 32 cores. We have noticed this with workload
> of a certain multiple threaded FEA (Finite Element Analysis) solvers.
> We reported this problem couple of years ago which eventually resulted in
> a new api for private futexes to avoid mmap_sem. The false sharing on
> the global futex hash was put off pending glibc changes to accomodate
> the futex private apis. Now that the glibc changes are in, and
> multicore is more prevalent, maybe it is time to fix this problem.
>
> The root cause of the problem is a global futex hash table even for process
> private futexes. Process private futexes can be hashed on process private
> hash tables, avoiding the global hash and a longer hash table walk when
> there are a lot more futexes in the workload. However, this results in an
> addition of one extra pointer to the mm_struct. Hence, this implementation
> of a process private hash table is based off a config option, which can be
> turned off for smaller core count systems. Furthermore, a subsequent patch
> will introduce a sysctl to dynamically turn on private futex hash tables.
>
> We found this patch to improve the runtime of a certain FEA solver by about
> 15% on a 32 core vSMP system.
>
> Signed-off-by: Ravikiran Thirumalai <kiran@scalex86.org>
> Signed-off-by: Shai Fultheim <shai@scalex86.org>
>

First incantation of PRIVATE_FUTEXES had process private hash table

http://lkml.org/lkml/2007/3/15/230

I dont remember objections at that time, maybe it was going to slow down small
users of these PRIVATE_FUTEXES, ie processes that will maybe use one futex_wait()
in their existence, because they'll have to allocate their private hash table
and populate it.

So I dropped parts about NUMA and private hash tables to get PRIVATE_FUTEXES into mainline.

http://lwn.net/Articles/229668/

Did you tried to change FUTEX_HASHBITS instead, since current value is really really
ridiculous ?

You could also try to adapt this patch to current kernels :

http://linux.derkeiler.com/Mailing-Lists/Kernel/2007-03/msg06504.html

[PATCH 3/3] FUTEX : NUMA friendly global hashtable

On NUMA machines, we should get better performance using a big futex
hashtable, allocated with vmalloc() so that it is spreaded on several nodes.

I chose a static size of four pages. (Very big NUMA machines have 64k page
size)