Commit 6de45f91 authored by Tim Smith's avatar Tim Smith Committed by Anthony PERARD
Browse files

xen-block: improve batching behaviour 



When I/O consists of many small requests, performance is improved by
batching them together in a single io_submit() call. When there are
relatively few requests, the extra overhead is not worth it. This
introduces a check to start batching I/O requests via blk_io_plug()/
blk_io_unplug() in an amount proportional to the number which were
already in flight at the time we started reading the ring.

Signed-off-by: default avatarTim Smith <tim.smith@citrix.com>

Re-based and commit comment adjusted.

Signed-off-by: default avatarPaul Durrant <paul.durrant@citrix.com>
Acked-by: default avatarAnthony PERARD <anthony.perard@citrix.com>
Signed-off-by: default avatarAnthony PERARD <anthony.perard@citrix.com>
parent 1077bcac

hw/block/dataplane/xen-block.c

+35 −0
Original line number Diff line number Diff line
@@ -528,10 +528,18 @@ static int xen_block_get_request(XenBlockDataPlane *dataplane, 
    return 0;

}



/*

 * Threshold of in-flight requests above which we will start using

 * blk_io_plug()/blk_io_unplug() to batch requests.

 */

#define IO_PLUG_THRESHOLD 1



static void xen_block_handle_requests(XenBlockDataPlane *dataplane)

{

    RING_IDX rc, rp;

    XenBlockRequest *request;

    int inflight_atstart = dataplane->requests_inflight;

    int batched = 0;



    dataplane->more_work = 0;
@@ -540,6 +548,18 @@ static void xen_block_handle_requests(XenBlockDataPlane *dataplane) 
    xen_rmb(); /* Ensure we see queued requests up to 'rp'. */



    xen_block_send_response_all(dataplane);

    /*

     * If there was more than IO_PLUG_THRESHOLD requests in flight

     * when we got here, this is an indication that there the bottleneck

     * is below us, so it's worth beginning to batch up I/O requests

     * rather than submitting them immediately. The maximum number

     * of requests we're willing to batch is the number already in

     * flight, so it can grow up to max_requests when the bottleneck

     * is below us.

     */

    if (inflight_atstart > IO_PLUG_THRESHOLD) {

        blk_io_plug(dataplane->blk);

    }

    while (rc != rp) {

        /* pull request from ring */

        if (RING_REQUEST_CONS_OVERFLOW(&dataplane->rings.common, rc)) {
@@ -585,7 +605,22 @@ static void xen_block_handle_requests(XenBlockDataPlane *dataplane) 
            continue;

        }



        if (inflight_atstart > IO_PLUG_THRESHOLD &&

            batched >= inflight_atstart) {

            blk_io_unplug(dataplane->blk);

        }

        xen_block_do_aio(request);

        if (inflight_atstart > IO_PLUG_THRESHOLD) {

            if (batched >= inflight_atstart) {

                blk_io_plug(dataplane->blk);

                batched = 0;

            } else {

                batched++;

            }

        }

    }

    if (inflight_atstart > IO_PLUG_THRESHOLD) {

        blk_io_unplug(dataplane->blk);

    }



    if (dataplane->more_work &&