linux-kernel - Re: dramatic I/O slowdown after upgrading 2.6.38->3.0+

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Date:	Tue, 10 Apr 2012 10:00:38 +0400
From:	Michael Tokarev <mjt@....msk.ru>
To:	Dave Chinner <david@...morbit.com>
CC:	Jan Kara <jack@...e.cz>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: dramatic I/O slowdown after upgrading 2.6.38->3.0+

On 10.04.2012 06:26, Dave Chinner wrote:

> Barriers. Turn them off, and see if that fixes your problem.

Thank you Dave for a hint.  And nope, that's not it, not at all... ;)
While turning off barriers helps a tiny bit, to gain a few %% from
the huge slowdown, it does not cure the issue.

Meanwhile, I observed the following:

1) the issue persists on more recent kernels too, I tried 3.3
   and it is also as slow as 3.0.

2) at least 2.6.38 kernel works fine, as fast as 2.6.32, I'll
   try 2.6.39 next.

   I updated $subject accordingly.

3) the most important thing I think: this is general I/O speed
   issue.  Here's why:

  2.6.38:
  # dd if=/dev/sdb of=/dev/null bs=1M iflag=direct count=100
  100+0 records in
  100+0 records out
  104857600 bytes (105 MB) copied, 1.73126 s, 60.6 MB/s

  3.0:
  # dd if=/dev/sdb of=/dev/null bs=1M iflag=direct count=100
  100+0 records in
  100+0 records out
  104857600 bytes (105 MB) copied, 29.4508 s, 3.6 MB/s

That's about 20 times difference on direct read from the
same - idle - device!!

Preparing for another bisect attempt, slowly.....

Thank you!

/mjt
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