On 01/09/2011 02:35 PM, Michael Zugelder wrote: Hi, > Just to be clear: This isn't about a dm-crypt encrypted disk being > slower that an unencrypted disk. It's about the access time of the same > encrypted disk rising to >= 10ms, which is at least an order of > magnitude for an SSD. I suspect with an HDD, the difference isn't much > noticeable. yep, it was reported several times, thanks for that bisect. We will need to retest it with per-cpu dm-crypt patches (should be in linux-next soon) but I expect that this problem remains. > I filed a bugreport on the Ubuntu bugtracker with a few pretty graphs, > vmstat and LatencyTOP output. If that would be useful, see > https://bugs.launchpad.net/ubuntu/+source/linux/+bug/653611 . (Maybe Ubuntu people can forward issue to upstream list next time?) > A bisect between .32 and .33 revealed some more information. I arrived > at 79da064, which is a revert of fb1e753 (block: improve > queue_should_plug() by looking at IO depths), so I compiled fb1e753. > With this 2.6.31 kernel, the problem was gone. One step back to 1f98a13, > the access time was high again. > > So, it seems that fb1e753 (block: improve queue_should_plug() by looking > at IO depths) improves dm-crypt access time on SSDs (by a factor of over > 40 here), but (as in the revert commit stated) reduces sequential > throughput in some cases. I applied fb1e753 to 2.6.37 and the access > time went down to the .32 value. > > For me, it seems that dm-crypt interferes with queuing, which totally > destroys access time. Yes, dm-crypt clones requests and process them in its own queue so there is some interference. I am not sure if this is fixable by block layer only patch (like fb1e753), Maybe dm-crypt should call unplug explicitly, flush crypt workqueue explicitly or something like that... dunno Jens, do you have an idea how to improve it and not lose throughput? Milan -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
