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

[Manfred Spraul <manfred@colorfullife.com>]

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