On Tue, Dec 20, 2011 at 12:32:41AM +0800, Jan Kara wrote:

On Wed 14-12-11 14:36:25, Wu Fengguang wrote:

quoted

quoted

  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@kvack.org.  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@kvack.org"> email@kvack.org </a>