Re: [dm-devel] How to track down abysmal performance ata - raid1 - crypto - vg/lv - xfs

[Dominik Brodowski <linux dominikbrodowski net>]

On Wed, Aug 04, 2010 at 07:18:03AM -0400, Christoph Hellwig wrote:
> On Wed, Aug 04, 2010 at 12:25:26PM +0200, Dominik Brodowski wrote:
> > Hey,
> > 
> > many thanks for your feedback. It seems the crypto step is the culprit:
> > 
> > Reading 1.1 GB with dd, iflag=direct, bs=8k:
> > 
> > /dev/sd*                35.3 MB/s       ( 90 %)
> > /dev/md*                39.1 MB/s       (100 %)
> > /dev/mapper/md*_crypt    3.9 MB/s       ( 10 %)
> > /dev/mapper/vg1-*        3.9 MB/s       ( 10 %)
> > 
> > The "good" news: it also happens on my notebook, even though it has a
> > different setup (no raid, disk -> lv/vg -> crypt). On my notebook, I'm
> > more than happy to test out different kernel versions, patches etc.
> > 
> > /dev/sd*                17.7 MB/s       (100 %)
> > /dev/mapper/vg1-*       16.2 MB/s       ( 92 %)
> > /dev/mapper/*_crypt      3.1 MB/s       ( 18 %)
> 
> The good news is that you have it tracked down, the bad news is that
> I know very little about dm-crypt.  Maybe the issue is the single
> threaded decryption in dm-crypt?  Can you check how much CPU time
> the dm crypt kernel thread uses?

2 CPUs overall:
Cpu(s):  1.0%us,  5.7%sy,  0.0%ni, 44.8%id, 47.0%wa,  0.0%hi,  1.5%si, 0.0%st

Thanks & best,
	Dominik