Messages in this thread | | | Date | Thu, 19 Feb 2004 13:01:30 +1100 | From | Nick Piggin <> | Subject | Re: IO scheduler, queue depth, nr_requests |
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Miquel van Smoorenburg wrote:
>On Thu, 19 Feb 2004 02:24:31, Nick Piggin wrote: > >>Miquel van Smoorenburg wrote: >> >> >>>I found out what causes this. It's get_request_wait(). >>> >>>When the request queue is full, and a new request needs to be created, >>>__make_request() blocks in get_request_wait(). >>> >>>Another process wakes up first (pdflush / process submitting I/O itself / >>>xfsdatad / etc) and sends the next bio's to __make_request(). >>>In the mean time some free requests have become available, and the bios >>>are merged into a new request. Those requests are submitted to the device. >>> >>>Then, get_request_wait() returns but the bio is not mergeable anymore - >>>and that results in a backwards seek, severely limiting the I/O rate. >>> >>> >>The "batching" logic there should allow a process to submit >>a number of requests even above the nr_requests limit to >>prevent this interleave and context switching. >> >>Are you using tagged command queueing? What depth? >> > >No, I'm not using tagged command queueing. The 3ware controller is not a >real scsi controller, the driver just emulates one. It's a raid5 controller >that drives SATA disks. It has an internal request queue ("can_queu") >of 254 outstanding commands. >
This is what I mean by tagged command queueing.
> Because that is way bigger than nr_requests >this happens - if I set nr_requests to 512, the problem goes away. But >that shouldn't happen ;) > >
What shouldn't happen?
>I'm preparing a proof-of-concept patch now, if it works and I don't wedge >the remote machine I'm testing this on I'll post it in a few minutes. > >
I'm not very happy with forcing a process to sleep _after_ it has submitted a request... but I'd be interested to see exactly what your patch does.
By far the best option is to use appropriately sized queues. The below patch is a start, but it unfortunately doesn't help drivers which use private queueing implementations.
http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.3/2.6.3-mm1/broken-out/scale-nr_requests.patch
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