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authorTim Chen <tim.c.chen@linux.intel.com>2010-08-18 15:00:27 -0700
committerGreg Kroah-Hartman <gregkh@suse.de>2010-08-26 16:46:32 -0700
commitf5d76aba51998b28810ae026a30577bd8d2abae5 (patch)
tree0366b77b6e5fc32adc2938a416d184c96456a57e
parenta87da791eaa200a75a47fabc4a39eaa32e42deca (diff)
mutex: Improve the scalability of optimistic spinning
commit 9d0f4dcc5c4d1c5dd01172172684a45b5f49d740 upstream. There is a scalability issue for current implementation of optimistic mutex spin in the kernel. It is found on a 8 node 64 core Nehalem-EX system (HT mode). The intention of the optimistic mutex spin is to busy wait and spin on a mutex if the owner of the mutex is running, in the hope that the mutex will be released soon and be acquired, without the thread trying to acquire mutex going to sleep. However, when we have a large number of threads, contending for the mutex, we could have the mutex grabbed by other thread, and then another ……, and we will keep spinning, wasting cpu cycles and adding to the contention. One possible fix is to quit spinning and put the current thread on wait-list if mutex lock switch to a new owner while we spin, indicating heavy contention (see the patch included). I did some testing on a 8 socket Nehalem-EX system with a total of 64 cores. Using Ingo's test-mutex program that creates/delete files with 256 threads (http://lkml.org/lkml/2006/1/8/50) , I see the following speed up after putting in the mutex spin fix: ./mutex-test V 256 10 Ops/sec 2.6.34 62864 With fix 197200 Repeating the test with Aim7 fserver workload, again there is a speed up with the fix: Jobs/min 2.6.34 91657 With fix 149325 To look at the impact on the distribution of mutex acquisition time, I collected the mutex acquisition time on Aim7 fserver workload with some instrumentation. The average acquisition time is reduced by 48% and number of contentions reduced by 32%. #contentions Time to acquire mutex (cycles) 2.6.34 72973 44765791 With fix 49210 23067129 The histogram of mutex acquisition time is listed below. The acquisition time is in 2^bin cycles. We see that without the fix, the acquisition time is mostly around 2^26 cycles. With the fix, we the distribution get spread out a lot more towards the lower cycles, starting from 2^13. However, there is an increase of the tail distribution with the fix at 2^28 and 2^29 cycles. It seems a small price to pay for the reduced average acquisition time and also getting the cpu to do useful work. Mutex acquisition time distribution (acq time = 2^bin cycles): 2.6.34 With Fix bin #occurrence % #occurrence % 11 2 0.00% 120 0.24% 12 10 0.01% 790 1.61% 13 14 0.02% 2058 4.18% 14 86 0.12% 3378 6.86% 15 393 0.54% 4831 9.82% 16 710 0.97% 4893 9.94% 17 815 1.12% 4667 9.48% 18 790 1.08% 5147 10.46% 19 580 0.80% 6250 12.70% 20 429 0.59% 6870 13.96% 21 311 0.43% 1809 3.68% 22 255 0.35% 2305 4.68% 23 317 0.44% 916 1.86% 24 610 0.84% 233 0.47% 25 3128 4.29% 95 0.19% 26 63902 87.69% 122 0.25% 27 619 0.85% 286 0.58% 28 0 0.00% 3536 7.19% 29 0 0.00% 903 1.83% 30 0 0.00% 0 0.00% I've done similar experiments with 2.6.35 kernel on smaller boxes as well. One is on a dual-socket Westmere box (12 cores total, with HT). Another experiment is on an old dual-socket Core 2 box (4 cores total, no HT) On the 12-core Westmere box, I see a 250% increase for Ingo's mutex-test program with my mutex patch but no significant difference in aim7's fserver workload. On the 4-core Core 2 box, I see the difference with the patch for both mutex-test and aim7 fserver are negligible. So far, it seems like the patch has not caused regression on smaller systems. Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Frederic Weisbecker <fweisbec@gmail.com> LKML-Reference: <1282168827.9542.72.camel@schen9-DESK> Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
-rw-r--r--kernel/sched.c10
1 files changed, 9 insertions, 1 deletions
diff --git a/kernel/sched.c b/kernel/sched.c
index 63b4a14682fa..6d0dbebc2d7f 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3694,8 +3694,16 @@ int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner)
/*
* Owner changed, break to re-assess state.
*/
- if (lock->owner != owner)
+ if (lock->owner != owner) {
+ /*
+ * If the lock has switched to a different owner,
+ * we likely have heavy contention. Return 0 to quit
+ * optimistic spinning and not contend further:
+ */
+ if (lock->owner)
+ return 0;
break;
+ }
/*
* Is that owner really running on that cpu?