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Date: Thu, 31 Jan 2013 10:39:31 +0100 From: Ingo Molnar <mingo@...nel.org> To: Yuanhan Liu <yuanhan.liu@...ux.intel.com> Cc: linux-kernel@...r.kernel.org, David Howells <dhowells@...hat.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Andrew Morton <akpm@...ux-foundation.org>, Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Steven Rostedt <rostedt@...dmis.org> Subject: Re: [PATCH] rwsem-spinlock: let rwsem write lock stealable * Yuanhan Liu <yuanhan.liu@...ux.intel.com> wrote: > We(Linux Kernel Performance project) found a regression introduced by > commit 5a50508, which just convert all mutex lock to rwsem write lock. > The semantics is same, but the results is quite huge in some cases. > After investigation, we found the root cause: mutex support lock > stealing. Here is the link for the detailed regression report: > https://lkml.org/lkml/2013/1/29/84 > > Ingo suggests to add write lock stealing to rwsem as well: > "I think we should allow lock-steal between rwsem writers - that > will not hurt fairness as most rwsem fairness concerns relate to > reader vs. writer fairness" > > I then tried it with rwsem-spinlock first as I found it much easier to > implement it than lib/rwsem.c. And here I sent out this patch first for > comments. I'd try lib/rwsem.c later once the change to rwsem-spinlock > is OK to you guys. > > With this patch, we got a double performance increase in one test box > with following aim7 workfile: > FILESIZE: 1M > POOLSIZE: 10M > 10 fork_test > > some /usr/bin/time output w/o patch some /usr/bin/time_output with patch > ---------------------------------------------------------------------------- > Percent of CPU this job got: 369% Percent of CPU this job got: 537% > Voluntary context switches: 640595016 Voluntary context switches: 157915561 > ---------------------------------------------------------------------------- > You will see we got a 45% increase of CPU usage and saves about 3/4 > voluntary context switches. > > > Here is the .nr_running filed for all CPUs from /proc/sched_debug. > > output w/o this patch: > ---------------------- > cpu 00: 0 0 ... 0 0 0 0 0 0 0 1 0 1 .... 0 0 > cpu 01: 0 0 ... 1 0 0 0 0 0 1 1 0 1 .... 0 0 > cpu 02: 0 0 ... 1 1 0 0 0 1 0 0 1 0 .... 1 1 > cpu 03: 0 0 ... 0 1 0 0 0 1 1 0 1 1 .... 0 0 > cpu 04: 0 1 ... 0 0 2 1 1 2 1 0 1 0 .... 1 0 > cpu 05: 0 1 ... 0 0 2 1 1 2 1 1 1 1 .... 0 0 > cpu 06: 0 0 ... 2 0 0 1 0 0 1 0 0 0 .... 0 0 > cpu 07: 0 0 ... 2 0 0 0 1 0 1 1 0 0 .... 1 0 > cpu 08: 0 0 ... 1 0 0 0 1 0 0 1 0 0 .... 0 1 > cpu 09: 0 0 ... 1 0 0 0 1 0 0 1 0 0 .... 0 1 > cpu 10: 0 0 ... 0 0 0 2 0 0 1 0 1 1 .... 1 2 > cpu 11: 0 0 ... 0 0 0 2 2 0 1 0 1 0 .... 1 2 > cpu 12: 0 0 ... 2 0 0 0 1 1 3 1 1 1 .... 1 0 > cpu 13: 0 0 ... 2 0 0 0 1 1 3 1 1 0 .... 1 1 > cpu 14: 0 0 ... 0 0 0 2 0 0 1 1 0 0 .... 1 0 > cpu 15: 0 0 ... 1 0 0 2 0 0 1 1 0 0 .... 0 0 > > output with this patch: > ----------------------- > cpu 00: 0 0 ... 1 1 2 1 1 1 2 1 1 1 .... 1 3 > cpu 01: 0 0 ... 1 1 1 1 1 1 2 1 1 1 .... 1 3 > cpu 02: 0 0 ... 2 2 3 2 0 2 1 2 1 1 .... 1 1 > cpu 03: 0 0 ... 2 2 3 2 1 2 1 2 1 1 .... 1 1 > cpu 04: 0 1 ... 2 0 0 1 0 1 3 1 1 1 .... 1 1 > cpu 05: 0 1 ... 2 0 1 1 0 1 2 1 1 1 .... 1 1 > cpu 06: 0 0 ... 2 1 1 2 0 1 2 1 1 1 .... 2 1 > cpu 07: 0 0 ... 2 1 1 2 0 1 2 1 1 1 .... 2 1 > cpu 08: 0 0 ... 1 1 1 1 1 1 1 1 1 1 .... 0 0 > cpu 09: 0 0 ... 1 1 1 1 1 1 1 1 1 1 .... 0 0 > cpu 10: 0 0 ... 1 1 1 0 0 1 1 1 1 1 .... 0 0 > cpu 11: 0 0 ... 1 1 1 0 0 1 1 1 1 2 .... 1 0 > cpu 12: 0 0 ... 1 1 1 0 1 1 0 0 0 1 .... 2 1 > cpu 13: 0 0 ... 1 1 1 0 1 1 1 0 1 2 .... 2 0 > cpu 14: 0 0 ... 2 0 0 0 0 1 1 1 1 1 .... 2 2 > cpu 15: 0 0 ... 2 0 0 1 0 1 1 1 1 1 .... 2 2 > ------------------------------------------------------------------------ > Where you can see that CPU is much busier with this patch. That looks really good - quite similar to how it behaved with mutexes, right? Does this recover most of the performance regression? Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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