From patchwork Thu Aug 25 21:29:27 2011 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Subject: [2/2] percpu_counter: Put a reasonable upper bound on percpu_counter_batch Date: Thu, 25 Aug 2011 11:29:27 -0000 From: Anton Blanchard X-Patchwork-Id: 111671 Message-Id: <20110826072927.5b4781f9@kryten> To: tytso@mit.edu, adilger.kernel@dilger.ca, eric.dumazet@gmail.com, tj@kernel.org Cc: linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org When testing on a 1024 thread ppc64 box I noticed a large amount of CPU time in ext4 code. ext4_has_free_blocks has a fast path to avoid summing every free and dirty block per cpu counter, but only if the global count shows more free blocks than the maximum amount that could be stored in all the per cpu counters. Since percpu_counter_batch scales with num_online_cpus() and the maximum amount in all per cpu counters is percpu_counter_batch * num_online_cpus(), this breakpoint grows at O(n^2). This issue will also hit with users of percpu_counter_compare which does a similar thing for one percpu counter. I chose to cap percpu_counter_batch at 1024 as a conservative first step, but we may want to reduce it further based on further benchmarking. Signed-off-by: Anton Blanchard --- -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Index: linux-2.6-work/lib/percpu_counter.c =================================================================== --- linux-2.6-work.orig/lib/percpu_counter.c 2011-07-31 20:37:12.580765739 +1000 +++ linux-2.6-work/lib/percpu_counter.c 2011-08-25 11:43:57.828695957 +1000 @@ -149,11 +149,15 @@ EXPORT_SYMBOL(percpu_counter_destroy); int percpu_counter_batch __read_mostly = 32; EXPORT_SYMBOL(percpu_counter_batch); +/* + * We set the batch at 2 * num_online_cpus(), with a minimum of 32 and + * a maximum of 1024. + */ static void compute_batch_value(void) { int nr = num_online_cpus(); - percpu_counter_batch = max(32, nr*2); + percpu_counter_batch = min(1024, max(32, nr*2)); } static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,