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>
---
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)
{
