Date:	Thu, 2 Apr 2009 13:19:49 -0400
From:	"Ross S. W. Walker" <RWalker@...allion.com>
To:	"Vladislav Bolkhovitin" <vst@...b.net>
Cc:	"James Bottomley" <James.Bottomley@...senPartnership.com>,
	<linux-scsi@...r.kernel.org>,
	"iSCSI Enterprise Target Developer List" 
	<iscsitarget-devel@...ts.sourceforge.net>,
	<linux-kernel@...r.kernel.org>,
	"Ross Walker" <rswwalker@...il.com>, <stgt@...r.kernel.org>,
	"scst-devel" <scst-devel@...ts.sourceforge.net>
Subject: RE: [Scst-devel] [Iscsitarget-devel] ISCSI-SCST	performance (with also IET and STGT data)

Vladislav Bolkhovitin wrote:
> 
> Think what you want and do what you want. You can even filter out all 
> e-mails from me, that's your right. But:
> 
> 1. As I wrote grouping threads into a single IO context doesn't explain 
> all the performance difference and finding out reasons for other's 
> performance problems isn't something I can afford at the moment.

No, not all the performance, but a substantial part of it, enough
so to say IET has a real performance issue when using CFQ scheduler.

> 2. CFQ doesn't have any processing latency and has never had. Learn to 
> understand what are your writing about and how to correctly express 
> yourself at first. You asked about that latency and I replied that there 
> is nothing to defeat.

CFQ pauses briefly before switching I/O contexts in order to make sure
it is giving as much bandwidth to a context before moving on. This is
documented. With a single I/O stream, or random I/O it won't be
noticeable, but for interleaved sequential I/O across multiple threads
with different I/O contexts it can be significant.

Not that Wikipedia is authorative: http://en.wikipedia.org/wiki/CFQ

It's right in the first paragraph:

"... While CFQ does not do explicit anticipatory IO scheduling, it
achieves the same effect of having good aggregate throughput for the
system as a whole, by allowing a process queue to idle at the end of
synchronous IO thereby "anticipating" further close IO from that
process. ..."

You can also check out the LXR:

This one in 2.6.18 kernels (RHEL) show a pause of HZ/10

http://lxr.linux.no/linux+v2.6.18/block/cfq-iosched.c#L30

So given a 10ms time slice, that would equate to ~1ms, in later
kernels it's defined as HZ/5 which can equate to ~2ms. These ms
delays can be an eternity for sequential I/O patterns.

> 3. SCST doesn't have any hooks into CFQ and not going to have in the 
> considerable future.

True, SCST doesn't have any hooks into CFQ, but your code modifies
block/blk-ioc.c to export the alloc_io_context(), which by default
is a private function, to allow your kernel based threads to set
their I/O contexts to the same group, therefore avoiding the delay
CFQ imposes on the switching of the I/O contexts between these
threads.

-Ross

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