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xen-block: improve batching behaviour

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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.smith@citrix.com>

Re-based and commit comment adjusted.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
This commit is contained in:
Tim Smith 2018-12-12 11:16:24 +00:00 committed by Anthony PERARD
parent 1077bcaccd
commit 6de45f9109
1 changed files with 35 additions and 0 deletions
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35
hw/block/dataplane/xen-block.c
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@ -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 &&