Re: Scalability requirements for sysv ipc (was: ipc: store ipcs intoIDRs)

[Nadia Derbey]

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


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