On Tue, Dec 20, 2011 at 12:32:41AM +0800, Jan Kara wrote: > On Wed 14-12-11 14:36:25, Wu Fengguang wrote: > > > This looks all inherently racy (which doesn't matter much as you suggest) > > > so I just wanted to suggest that if you used per-cpu counters you'd get > > > race-free and faster code at the cost of larger data structures and using > > > percpu_counter_add() instead of ++ (which doesn't seem like a big > > > complication to me). > > > > OK, here is the incremental patch to use per-cpu counters :) > Thanks! This looks better. I just thought you would use per-cpu counters > as defined in include/linux/percpu_counter.h and are used e.g. by bdi > stats. This is more standard for statistics in the kernel than using > per-cpu variables directly. Ah yes, I overlooked that facility! However the percpu_counter's ability to maintain and quickly retrieve the global value seems unnecessary feature/overheads for readahead stats, because here we only need to sum up the global value when the user requests it. If switching to percpu_counter, I'm afraid every readahead(1MB) event will lead to the update of percpu_counter global value (grabbing the spinlock) due to 1MB > some small batch size. This actually performs worse than the plain global array of values in the v1 patch. Thanks, Fengguang -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>