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SubjectScalability requirements for sysv ipc (was: ipc: store ipcs into IDRs)
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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