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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