linux-kernel - Re: per-cpu stats in block device: overkill?

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Message-ID: <494EF3AA.9040100@kernel.org>
Date:	Mon, 22 Dec 2008 10:55:54 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Jens Axboe <jens.axboe@...cle.com>,
	Jerome Marchand <jmarchan@...hat.com>,
	linux-kernel@...r.kernel.org,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: per-cpu stats in block device: overkill?

Hello, Rusty.

Rusty Russell wrote:
> Hi Jens, Tejun, Jerome,
> 
> I've been auditing alloc_per_cpu users, and got to genhd.  The code
> is fairly complex, but I can't help wondering if per-cpu counters
> are overkill.  After all, we have a single queue lock.

Yeah, maybe.

> The reason I care is that I'm changing alloc_per_cpu to use the
> static per-cpu area: at 40/80 bytes (32/64 bit) per stat, we'd be
> restricted to a few hundred disks unless the percpu area is enlarged
> (in current patches, a cmdline param).  Or, I can change genhd to
> use big_percpu_alloc which will use the current inefficient dynamic
> per-cpu system until we get dynamic per-cpu regions (if ever).

I'm working on local counter (local_t) allocator which is used to
replace percpu allocation in percpu_counter and used as basis for
percpu_ref which replaces module ref counting and will be used to
simplify block/char lifetime rules.

The local counter allocator allocates per-cpu pages and the space
overhead is minimal.  If per-cpu stats in genhd is necessary, I think
converting it to percpu local counter allocation should do it.

BTW, why make percpu area static?

Thanks.

-- 
tejun
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