linux-kernel - Re: [Scst-devel] ISCSI-SCST performance (with also IET and STGT data)

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Date:	Fri, 17 Apr 2009 22:11:09 +0400
From:	Vladislav Bolkhovitin <vst@...b.net>
To:	Bart Van Assche <bart.vanassche@...il.com>
CC:	scst-devel <scst-devel@...ts.sourceforge.net>,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: [Scst-devel] ISCSI-SCST performance (with also IET and STGT	data)

Bart Van Assche, on 04/04/2009 12:04 PM wrote:
> On Thu, Apr 2, 2009 at 7:16 PM, Vladislav Bolkhovitin <vst@...b.net> wrote:
>> Bart Van Assche, on 04/02/2009 12:14 AM wrote:
>>> I have repeated some of these performance tests for iSCSI over IPoIB
>>> (two DDR PCIe 1.0 ConnectX HCA's connected back to back). The results
>>> for the buffered I/O test with a block size of 512K (initiator)
>>> against a file of 1GB residing on a tmpfs filesystem on the target are
>>> as follows:
>>>
>>> write-test: iSCSI-SCST 243 MB/s; IET 192 MB/s.
>>> read-test: iSCSI-SCST 291 MB/s; IET 223 MB/s.
>>>
>>> And for a block size of 4 KB:
>>>
>>> write-test: iSCSI-SCST 43 MB/s; IET 42 MB/s.
>>> read-test: iSCSI-SCST 288 MB/s; IET 221 MB/s.
>> Do you have any thoughts why writes are so bad? It shouldn't be so..
> 
> By this time I have run the following variation of the 4 KB write test:
> * Target: iSCSI-SCST was exporting a 1 GB file residing on a tmpfs filesystem.
> * Initiator: two processes were writing 4 KB blocks as follows:
> dd if=/dev/zero of=/dev/sdb bs=4K seek=0 count=131072 oflag=sync &
> dd if=/dev/zero of=/dev/sdb bs=4K seek=131072 count=131072 oflag=sync &
> 
> Results:
> * Each dd process on the initiator was writing at a speed of 37.8
> MB/s, or a combined writing speed of 75.6 MB/s.
> * CPU load on the initiator system during the test: 2.0.
> * According to /proc/interrupts, about 38000 mlx4-comp-0 interrupts
> were triggered per second.
> 
> These results confirm that the initiator system was the bottleneck
> during the 4 KB write test, not the target system.

If so with oflag=direct you should have a performance gain, because you 
will eliminate a data copy.

> Bart.
> 
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