On Wed, Dec 21, 2011 at 09:29:36AM +0800, Wu Fengguang wrote:

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

quoted

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.

So use a custom batch size so that typical increments don't require
locking for every add. The bdi stat counters are an example of this
sort of setup to reduce lock contention on typical IO workloads as
concurrency increases.

All these stats have is a requirement for a different batch size to
avoid frequent lock grabs. The stats don't have to update the global
counter very often (only to prvent overflow!) so you count get away
with a batch size in the order of 2^30 without any issues....

We have a general per-cpu counter infrastructure - we should be
using it and improving it and not reinventing it a different way
every time we need a per-cpu counter.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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