Re: A question about NCQ

[Tejun Heo <htejun@gmail.com>]

zhao, forrest wrote:
> On Tue, 2006-05-16 at 19:49 +0900, Tejun Heo wrote:
>> I don't know the workload of iozone.  But NCQ shines when there are many
>> concurrent IOs in progress.  A good real world example would be busy
>> file-serving web server.  It generally helps if there are multiple IO
>> requests.  If iozone is single-threaded (IO-wise), try to run multiple
>> copies of them and compare the results.
>>
>> Also, you need to pay attention to IO schedule in use, IIRC as and cfq
>> are heavily optimized for single-queued devices and might not show the
>> best performance depending on workload.  For functionality test, I
>> usually use deadline.  It's simpler and usually doesn't get in the way,
>> which, BTW, may or may not translate into better performance.
>>
> Tejun,
> 
> I run iozone with 8 concurrent threads. From my understanding, NCQ
> should at least provide the same throughput as non-NCQ. But the attached
> test result showed that NCQ has the lower throughput compared with non-
> NCQ.
> 
> The io scheduler is anticipatory.
> The kernel without NCQ is 2.6.16-rc6, the kernel with NCQ is #upstream.
> 
> The current problem is that I don't know where the bottleneck is, block
> I/O layer, SCSI layer, device driver layer or hardware problem......

AFAIK, anticipatory doesn't interact very well with queued devices.  Can
you try with deadline?

-- 
tejun