Re: IO scheduler, queue depth, nr_requests

[Miquel van Smoorenburg <miquels@cistron.nl>]

On Tue, 17 Feb 2004 15:57:16, Miquel van Smoorenburg wrote:
> For some reason, when using LVM, write requests get queued out
> of order to the 3ware controller, which results in quite a bit
> of seeking and thus performance loss.
[..]
> Okay I repeated some earlier tests, and I added some debug code in
> several places.
> 
> I added logging to tw_scsi_queue() in the 3ware driver to log the
> start sector and length of each request. It logs something like:
> 3wdbg: id 119, lba = 0x2330bc33, num_sectors = 256
> 
> With a perl script, I can check if the requests are sent to the
> host in order. That outputs something like this:
> 
> Consecutive: start 1180906348, length 7936 sec (3968 KB), requests: 31
> Consecutive: start 1180906340, length 8 sec (4 KB), requests: 1
> Consecutive: start 1180914292, length 7936 sec (3968 KB), requests: 31
> Consecutive: start 1180914284, length 8 sec (4 KB), requests: 1
> Consecutive: start 1180922236, length 7936 sec (3968 KB), requests: 31
> Consecutive: start 1180922228, length 8 sec (4 KB), requests: 1
> Consecutive: start 1180930180, length 7936 sec (3968 KB), requests: 31
> 
> See, 31 requests in order, then one request "backwards", then 31 in order, etc.

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.

Wouldn't it be better to allow the request allocation and queue the
request, and /then/ put the process to sleep ? The queue will grow larger
than nr_requests, but it does that anyway.

Mike.