Subject: Re: [PATCH 2/2] ipc/sem: sem_lock with hysteresis

From: Davidlohr Bueso <[email-masked]>
Date: Tue, 28 Jun 2016 10:54:50 -0700

On Sat, 25 Jun 2016, Manfred Spraul wrote:

>- Only simple ops: patch has no impact (the first 10 semops do not matter)

Agreed.

>- sleeping complex ops: patch has no impact, we are always in complex mode
>- not sleeping complex ops: depends on the size of the array.
>With a 4.000 semaphore array, I see an improvement of factor 20.
>
>There is obviously one case where the patch causes a slowdown:
>- complex op, then 11 simple ops, then repeat.

Yeah, you reset the counter to COMPLEX_MODE_ENTER every time we do a
complexmode_enter(), so if you have interleaving of complex and simple
ops you could end up always taking the big lock. This is my main concern
and not an unusual scenario at all.

I wonder if we could be a bit less aggressive if already in complex_mode
and do something like:

if (sma->complex_mode > 0) {
   WRITE_ONCE(sma->complex_mode, min(complex_mode + 1, COMPLEX_MODE_ENTER));
   return;
}

and still be constrained by COMPLEX_MODE_ENTER... of course that has its own
additional overhead, albeit less contention on the big lock :/

>
>Perhaps: set COMPLEX_MODE_ENTER to 1 or 2, then allow to configure it
>from user space.

Nah, I don't think it's a good idea to expose such internals to userspace.

>Or do not merge the patch and wait until someone come with a profile
>that shows complexmode_enter().

Testing/benchmarking this patch is on my todo list mainly because I'm lacking
a decent box to test it on. But I'm not conformable having this one without
any numbers for say at least a rdbms benchmark. I'm very eager to add the
first patch (complex_mode) once the nits are settled, as it fixes a real bug.

Thanks,
Davidlohr