> -----Original Message-----
> From: Qemu-devel [mailto:qemu-devel-
> bounces+paul.durrant=citrix....@nongnu.org] On Behalf Of Tim Smith
> Sent: 07 September 2018 11:21
> To: qemu-devel@nongnu.org
> Subject: [Qemu-devel] [PATCH 1/3] Improve xen_disk 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: Tim Smith <tim.sm...@citrix.com>
Reviewed-by: Paul Durrant <paul.durr...@citrix.com>
> ---
> hw/block/xen_disk.c | 29 +++++++++++++++++++++++++++++
> 1 file changed, 29 insertions(+)
>
> diff --git a/hw/block/xen_disk.c b/hw/block/xen_disk.c
> index 36eff94f84..6cb40d66fa 100644
> --- a/hw/block/xen_disk.c
> +++ b/hw/block/xen_disk.c
> @@ -101,6 +101,9 @@ struct XenBlkDev {
> AioContext *ctx;
> };
>
> +/* 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 ioreq_reset(struct ioreq *ioreq)
> @@ -542,6 +545,8 @@ static void blk_handle_requests(struct XenBlkDev
> *blkdev)
> {
> RING_IDX rc, rp;
> struct ioreq *ioreq;
> + int inflight_atstart = blkdev->requests_inflight;
> + int batched = 0;
>
> blkdev->more_work = 0;
>
> @@ -550,6 +555,16 @@ static void blk_handle_requests(struct XenBlkDev
> *blkdev)
> xen_rmb(); /* Ensure we see queued requests up to 'rp'. */
>
> blk_send_response_all(blkdev);
> + /* If there was more than IO_PLUG_THRESHOLD ioreqs 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(blkdev->blk);
> + }
> while (rc != rp) {
> /* pull request from ring */
> if (RING_REQUEST_CONS_OVERFLOW(&blkdev->rings.common, rc)) {
> @@ -589,7 +604,21 @@ static void blk_handle_requests(struct XenBlkDev
> *blkdev)
> continue;
> }
>
> + if (inflight_atstart > IO_PLUG_THRESHOLD && batched >=
> inflight_atstart) {
> + blk_io_unplug(blkdev->blk);
> + }
> ioreq_runio_qemu_aio(ioreq);
> + if (inflight_atstart > IO_PLUG_THRESHOLD) {
> + if (batched >= inflight_atstart) {
> + blk_io_plug(blkdev->blk);
> + batched=0;
> + } else {
> + batched++;
> + }
> + }
> + }
> + if (inflight_atstart > IO_PLUG_THRESHOLD) {
> + blk_io_unplug(blkdev->blk);
> }
>
> if (blkdev->more_work && blkdev->requests_inflight < blkdev-
> >max_requests) {
>
