Messages in this thread | | | Date | Fri, 21 Mar 2008 10:41:47 +0100 | From | Manfred Spraul <> | Subject | Scalability requirements for sysv ipc (was: ipc: store ipcs into IDRs) |
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Hi all,
I noticed that sysv ipc now uses very special locking: first a global rw-semaphore, then within that semaphore rcu: > linux-2.6.25-rc3:/ipc/util.c: > struct kern_ipc_perm *ipc_lock(struct ipc_ids *ids, int id) > { > struct kern_ipc_perm *out; > int lid = ipcid_to_idx(id); > > down_read(&ids->rw_mutex); > > rcu_read_lock(); > out = idr_find(&ids->ipcs_idr, lid); ids->rw_mutex is a per-namespace (i.e.: usually global) semaphore. Thus ipc_lock writes into a global cacheline. Everything else is based on per-object locking, especially sysv sem doesn't contain a single global lock/statistic counter/... That can't be the Right Thing (tm): Either there are cases where we need the scalability (then using IDRs is impossible), or the scalability is never needed (then the remaining parts from RCU should be removed). I don't have a suitable test setup, has anyone performed benchmarks recently? Is sysv semaphore still important, or have all apps moved to posix semaphores/futexes? Nadia: Do you have access to a suitable benchmark?
A microbenchmark on a single-cpu system doesn't help much (except that 2.6.25 is around factor 2 slower for sysv msg ping-pong between two tasks compared to the numbers I remember from older kernels....)
-- Manfred
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