Messages in this thread | | | Date | Fri, 21 Mar 2008 13:45:23 +0100 | From | Nadia Derbey <> | Subject | Re: Scalability requirements for sysv ipc (was: ipc: store ipcs into IDRs) |
| |
Manfred Spraul wrote: > 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....) >
If I remember well, at that time I had used ctxbench and I wrote some other small scripts. And the results I had were around 2 or 3% slowdown, but I have to confirm that by checking in my archives.
I'll also have a look at the remaining RCU critical sections in the code.
Regards, Nadia
| |