Re: IO scheduler, queue depth, nr_requests

[Nick Piggin]

Miquel van Smoorenburg wrote:

> 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.

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?

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