[stgt] [Scst-devel] [Iscsitarget-devel] ISCSI-SCST performance (with also IET and STGT data)

Ross S. W. Walker RWalker at medallion.com
Thu Apr 2 19:19:49 CEST 2009

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


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


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