Re: Fine-grained locking for POSIX local sockets (UNIX domain sockets)

On Sun, May 07, 2006 at 05:41:53PM -0400, Kris Kennaway wrote:

static int
kern_sigtimedwait(struct thread *td, sigset_t waitset, ksiginfo_t *ksi,
struct timespec *timeout)
{
...
td->td_sigmask = savedmask;
SIGSETNAND(td->td_sigmask, waitset);
signotify(td);
error = msleep(&ps, &p->p_mtx, PPAUSE|PCATCH, "sigwait", hz);

i.e. several threads in the process are all sleeping for 1 tick and
then doing a thundering herd on the same proc lock when they wake up.

Oops, actually hz=0; it comes via sigwait(2). It's still a thundering
herd situation though.

I also profiled the sleepq_broadcast call to look at thundering herds
from wakeup(), and I found that the umtxq_chain is waking the
following distribution of CPUs at once:

Before patch:

Freq #CPUs
1111 1
366 2
136 3
54 4
17 5
2 6

After patch:

Freq #CPUs
1139 1
332 2
139 3
44 4
12 5
2 6

i.e. apparently not a large difference, but still a large proportion
of cases where multiple CPUs are woken at once on the same chain.

Kris

Attachment: pgpYpaNJewEcP.pgp
Description: PGP signature

Relevant Pages