Re: [GIT PULL] Ext3 latency fixes

[Jens Axboe]

On Sat, Apr 04 2009, Theodore Tso wrote:
> On Sat, Apr 04, 2009 at 08:01:08PM +0200, Jens Axboe wrote:
> > 
> > Unless you make all journal writes sync, ext3 fsync will always suck big
> > time. But I get your point.
> > 
> 
> We have already made all other journal writes sync when triggered by
> fsync() --- except for the commit record which is being written by
> sync_dirty_buffer().  I've verified this via blktrace.
> 
> However, you're right.  We do have an performance regression using the
> regression test provided in commit 78f707bf.  Taking the average of at
> least 5 runs, I found:
> 
> stock 2.6.29				 1687 ms
> 2.6.29 w/ ext3-latency-fixes		 7337 ms
> 2.6.29 w/ full-latency-fixes		13722 ms
> 
> "ext3-latency-fixes" are the patches which Linus has already pulled
> into the 2.6.29 tree.  "full-latency-fixes" are ext3-latency-fixes
> plus the sync_dirty_buffes() patch.

Ugh yes, not everyone will appreciate the better latency and for them an
8x slowdown is veeery painful.

> Looking at blktrace of stock 2.6.29 running the sqlite performance
> measurement, we see this:
> 
> 254,2    1       13     0.005120554  7183  Q   W 199720 + 8 [sqlite-fsync-te]
> 254,2    1       15     0.005666573  6947  Q   W 10277200 + 8 [kjournald]
> 254,2    1       16     0.005691576  6947  Q   W 10277208 + 8 [kjournald]
> 254,2    1       18     0.006145684  6947  Q   W 10277216 + 8 [kjournald]
> 254,2    1       21     0.006685348  7183  Q   W 199720 + 8 [sqlite-fsync-te]
> 254,2    1       24     0.007162644  6947  Q   W 10277224 + 8 [kjournald]
> 254,2    1       25     0.007187857  6947  Q   W 10277232 + 8 [kjournald]
> 254,2    0       27     0.007616473  6947  Q   W 10277240 + 8 [kjournald]
> 
> Looking at a blktrace of 2.6.29 plus the full-latency-fixes, we see this:
> 
> 254,2    0       13     0.013744556  7205  Q  WS 199208 + 8 [sqlite-fsync-te]
> 254,2    0       16     0.019270748  6965  Q  WS 10301896 + 8 [kjournald]
> 254,2    0       17     0.019400024  6965  Q  WS 10301904 + 8 [kjournald]
> 254,2    1       23     0.019892824  6965  Q  WS 10301912 + 8 [kjournald]
> 254,2    0       20     0.020450995  7205  Q  WS 199208 + 8 [sqlite-fsync-te]
> 254,2    1       26     0.025954909  6965  Q  WS 10301920 + 8 [kjournald]
> 254,2    1       27     0.026084814  6965  Q  WS 10301928 + 8 [kjournald]
> 254,2    0       24     0.026612884  6965  Q  WS 10301936 + 8 [kjournald]
> 
> Looking at the deltas between the two iterations of the sqlite
> analogue, we see that stock 2.6.29 is 1.56ms, where as with the full
> latency fixes, it's 6.70ms. 

It would be interesting with the full set of information, like unplug
and merge. The above definitely shows longer between queue-to-queue, I
wonder what that is due to.

> However, the full latency fixes all the writes are synchronous, so it
> must be the case that the delays are caused by the fact that queue is
> getting implicitly unplugged after the synchronous write, and the
> problem is no longer the mixing of WRITE and WRITE_SYNC requests as
> posted in the commit log for 78f707bf.  If we remove the automatic
> unplug for WRITE_SYNC requests, and add an explicit unplug where it is
> needed, that should fix the performance regression for this particular
> sqlite test case.  (Which isn't a throughput issue, since the test is
> basically fopen/write/fsync/fclose over and over again, with no
> background load.)

We definitely should add a WRITE_SYNC variant that doesn't unplug, for
the cases where you know you are going to submit more than one IO before
waiting. Whether that should just be WRITE_SYNC or we should add a
WRITE_SYNC_UNPLUG, I don't have a really strong preference either way.

But if the delays are due to premature unplugging, it should be visible
in the trace. Care to send the "full" thing?

> The scenario which Linus and I had been focused on is one where there
> was a heavy background load writing asynchronously.  We do want to
> make sure that a series of fsync() calls in a tight loop is also
> reasonable as well, so Jens, I do think you are absolutely right that
> this is something that we need to pay attention to.

Most definitely!

> I had missed the commit 78f707bf when you originally submitted it, so
> I didn't do this test before submitting the patch.  And I guess you
> had missed my patch proposal to LKML, and I didn't think to explicitly
> CC you on my patches.  Apologies for the communication faults, but
> hopefully we can fix this performance issue for both cases and get
> these problems behind us.

We still have a long cycle ahead of us, so I'm sure we can get it nailed
down before release.

-- 
Jens Axboe

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