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....)