Re: direct-io regression [Was: How to track down abysmalperformance ata - raid1 - crypto - vg/lv - xfs]

From: Dominik Brodowski
Date: Thu Aug 05 2010 - 11:36:09 EST


On Thu, Aug 05, 2010 at 07:32:40AM -0400, Chris Mason wrote:
> But, I'm surprised your drive is doing 8K dio reads at 16MB/s, that
> seems a little high.

Well, that's what it does:

# $ dd if=/dev/mapper/vg0-home_crypt of=/dev/zero iflag=direct bs=8k count=131072 seek=131072
# 131072+0 records in
# 131072+0 records out
# 1073741824 bytes (1.1 GB) copied, 62.0177 s, 17.3 MB/s

On Thu, Aug 05, 2010 at 08:36:49AM -0400, Josef Bacik wrote:
> Hrm, I made sure there were no perf regressions when I wast testing this stuff,
> though I think I only tested xfs and ext4.

For this test, I'm not doing dio on filesystem level, but on block level
(/dev/mapper/vg0-*_crypt). It seems that dm-crypt creates such offending
holes, which cause this huge performance drop.

> Originally I had a test where if we
> provided our own submit_io, so maybe as a workaround just make
> if (dio->final_block_in_bio != dio->cur_page_block ||
> cur_offset != bio_next_offset)
> look like this
> if (dio->final_block_in_bio != dio->cur_page_block ||
> (dio->submit_io && cur_offset != bio_next_offset))

Tested-by: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>

With this fix, I get proper speeds when doing dio reads from
/dev/mapper/vg0-*_crypt; see the 17.3 MB/s above. Most strangely,
also accesing /dev/mapper/vg0-* (un-encrypted) and the raw
device at /dev/sda* speeds up (to up to 28 MB/s). Was only seeing around
16 to 18 MB/s without this patch for unencrypted access.

> I know why it could cause a problem, but this change shouldn't be
> causing a 400% regression.

Well, it seems to cause -- at least on my notebook -- a 150% regression on
unencrypted LVM2 access; and this > 400% on encrypted LVM2 access...

> I suspect something else is afoot here.

There is, probably. But the fix you propose helps a lot, already.

Thanks & best,

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