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From: Heinz Diehl <htd@fancy-poultry.org>
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Subject: Re: [dm-crypt] dm-crypt: Performance Regression 2.6.37 -> 2.6.38-rc8
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On 09.03.2011, Mario 'BitKoenig' Holbe wrote: 

> dm-crypt in 2.6.38 changed to per-CPU workqueues to increase it's
> performance by parallelizing encryption to multiple CPUs.
> This modification seems to cause (massive) performance drops for
> multiple parallel dm-crypt instances...

Not that it adresses your problem, but in my case, this patch is the
best one in the last months. On my good old primary LUKS/dmcrypt
encrypted partitions, on an AMD Quadcore, it boosts performance from 
45 MB/s w/o the patch up to 87 MB/s with it. That's just an incredibly 
huge performance boost.