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