linux-kernel - Re: [PATCH 6/9] readahead: add /debug/readahead/stats

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Date:	Wed, 21 Dec 2011 09:29:36 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	Jens Axboe <axboe@...nel.dk>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/9] readahead: add /debug/readahead/stats

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
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