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

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Date:	Mon, 22 Dec 2008 10:49:43 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Tejun Heo <tj@...nel.org>, Jerome Marchand <jmarchan@...hat.com>,
	linux-kernel@...r.kernel.org,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: per-cpu stats in block device: overkill?

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.

  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).

Cheers,
Rusty.
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