[linux-lvm] Re: IO scheduler, queue depth, nr_requests

Miquel van Smoorenburg miquels at cistron.nl
Thu Feb 19 09:00:05 UTC 2004

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. 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 ;)

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.


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