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[Qemu-devel] [PATCH 1/3] Improve xen_disk batching behaviour
From: |
Tim Smith |
Subject: |
[Qemu-devel] [PATCH 1/3] Improve xen_disk batching behaviour |
Date: |
Fri, 2 Nov 2018 10:00:59 +0000 |
User-agent: |
StGit/0.17.1-dirty |
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 <address@hidden>
---
hw/block/xen_disk.c | 30 ++++++++++++++++++++++++++++++
1 file changed, 30 insertions(+)
diff --git a/hw/block/xen_disk.c b/hw/block/xen_disk.c
index 36eff94f84..cb2881b7e6 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 +604,22 @@ 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)
{
- [Qemu-devel] [PATCH 0/3] Performance improvements for xen_disk v2, Tim Smith, 2018/11/02
- [Qemu-devel] [PATCH 3/3] Avoid repeated memory allocation in xen_disk, Tim Smith, 2018/11/02
- [Qemu-devel] [PATCH 2/3] Improve xen_disk response latency, Tim Smith, 2018/11/02
- [Qemu-devel] [PATCH 1/3] Improve xen_disk batching behaviour,
Tim Smith <=
- [Qemu-devel] xen_disk qdevification (was: [PATCH 0/3] Performance improvements for xen_disk v2), Kevin Wolf, 2018/11/02
- Re: [Qemu-devel] xen_disk qdevification (was: [PATCH 0/3] Performance improvements for xen_disk v2), Paul Durrant, 2018/11/02
- Re: [Qemu-devel] xen_disk qdevification (was: [PATCH 0/3] Performance improvements for xen_disk v2), Kevin Wolf, 2018/11/02
- Re: [Qemu-devel] xen_disk qdevification, Markus Armbruster, 2018/11/05
- Re: [Qemu-devel] xen_disk qdevification, Paul Durrant, 2018/11/05
- Re: [Qemu-devel] xen_disk qdevification, Paul Durrant, 2018/11/08
- Re: [Qemu-devel] xen_disk qdevification, Kevin Wolf, 2018/11/08
- Re: [Qemu-devel] xen_disk qdevification, Paul Durrant, 2018/11/08