From: Davidlohr Bueso <dave@stgolabs.net>
To: Manfred Spraul <manfred@colorfullife.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>,
Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
"H. Peter Anvin" <hpa@zytor.com>,
Peter Zijlstra <peterz@infradead.org>,
Andrew Morton <akpm@linux-foundation.org>,
LKML <linux-kernel@vger.kernel.org>,
linux-next@vger.kernel.org, 1vier1@web.de,
felixh@informatik.uni-bremen.de
Subject: Re: [PATCH 2/2] ipc/sem: sem_lock with hysteresis
Date: Tue, 28 Jun 2016 10:54:50 -0700 [thread overview]
Message-ID: <20160628175450.GA19886@linux-80c1.suse> (raw)
In-Reply-To: <fb23c7da-52a3-5e90-6c45-10e33d80417b@colorfullife.com>
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
next prev parent reply other threads:[~2016-06-28 17:55 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-15 5:23 linux-next: manual merge of the akpm-current tree with the tip tree Stephen Rothwell
2016-06-18 19:39 ` Manfred Spraul
2016-06-18 20:02 ` [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race Manfred Spraul
2016-06-18 20:02 ` [PATCH 2/2] ipc/sem: sem_lock with hysteresis Manfred Spraul
2016-06-21 20:29 ` Davidlohr Bueso
2016-06-25 17:37 ` Manfred Spraul
2016-06-28 17:54 ` Davidlohr Bueso [this message]
2016-06-20 23:04 ` [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race Andrew Morton
2016-06-23 18:55 ` Manfred Spraul
2016-06-21 0:30 ` Davidlohr Bueso
2016-06-23 19:22 ` Manfred Spraul
2016-06-28 5:27 ` Davidlohr Bueso
2016-06-30 19:28 ` Manfred Spraul
2016-07-01 16:52 ` Davidlohr Bueso
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