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 ______________________________________________________________________ This e-mail, and any attachments thereto, is intended only for use by the addressee(s) named herein and may contain legally privileged and/or confidential information. If you are not the intended recipient of this e-mail, you are hereby notified that any dissemination, distribution or copying of this e-mail, and any attachments thereto, is strictly prohibited. If you have received this e-mail in error, please immediately notify the sender and permanently delete the original and any copy or printout thereof. -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
