Re: IO scheduler, queue depth, nr_requests

[Nick Piggin <piggin@cyberone.com.au>]

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?