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SubjectRe: dramatic I/O slowdown after upgrading 2.6.38->3.0+
On 04/10/2012 08:43 PM, Jan Kara wrote:
> On Tue 10-04-12 10:00:38, Michael Tokarev wrote:
>> 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!!
> Huh, that's a huge difference for such a trivial load. So we can rule out
> filesystems, writeback, mm. I also wouldn't think it's IO scheduler but
> you can always check by comparing dd numbers after
> echo none >/sys/block/sdb/queue/scheduler

s/none/noop
you meant noop, of course?


Suresh


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