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    SubjectRe: 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.
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