Messages in this thread | | | Date | Thu, 19 Feb 2004 00:52:43 +0100 | From | Miquel van Smoorenburg <> | Subject | Re: IO scheduler, queue depth, nr_requests |
| |
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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |