[dm-crypt] [dm-devel] dm-crypt performance
Alasdair G Kergon
agk at redhat.com
Tue Mar 26 13:27:13 CET 2013
[Adding dm-crypt + linux-kernel]
On Mon, Mar 25, 2013 at 11:47:22PM -0400, Mikulas Patocka wrote:
> I performed some dm-crypt performance tests as Mike suggested.
> It turns out that unbound workqueue performance has improved somewhere
> between kernel 3.2 (when I made the dm-crypt patches) and 3.8, so the
> patches for hand-built dispatch are no longer needed.
> For RAID-0 composed of two disks with total throughput 260MB/s, the
> unbound workqueue performs as well as the hand-built dispatch (both
> sustain the 260MB/s transfer rate).
> For ramdisk, unbound workqueue performs better than hand-built dispatch
> (620MB/s vs 400MB/s). Unbound workqueue with the patch that Mike suggested
> (git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq.git) improves
> performance slighlty on ramdisk compared to 3.8 (700MB/s vs. 620MB/s).
> However, there is still the problem with request ordering. Milan found out
> that under some circumstances parallel dm-crypt has worse performance than
> the previous dm-crypt code. I found out that this is not caused by
> deficiencies in the code that distributes work to individual processors.
> Performance drop is caused by the fact that distributing write bios to
> multiple processors causes the encryption to finish out of order and the
> I/O scheduler is unable to merge these out-of-order bios.
> The deadline and noop schedulers perform better (only 50% slowdown
> compared to old dm-crypt), CFQ performs very badly (8 times slowdown).
> If I sort the requests in dm-crypt to come out in the same order as they
> were received, there is no longer any slowdown, the new crypt performs as
> well as the old crypt, but the last time I submitted the patches, people
> objected to sorting requests in dm-crypt, saying that the I/O scheduler
> should sort them. But it doesn't. This problem still persists in the
> current kernels.
> For best performance we could use the unbound workqueue implementation
> with request sorting, if people don't object to the request sorting being
> done in dm-crypt.
On Tue, Mar 26, 2013 at 02:52:29AM -0400, Christoph Hellwig wrote:
> FYI, XFS also does it's own request ordering for the metadata buffers,
> because it knows the needed ordering and has a bigger view than than
> than especially CFQ. You at least have precedence in a widely used
> subsystem for this code.
So please post this updated version of the patches for a wider group of
people to try out.
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