@@ -92,7 +92,11 @@ static void qemu_laio_process_completion(struct qemu_laiocb *laiocb)
laiocb->ret = ret;
if (laiocb->co) {
- qemu_coroutine_enter(laiocb->co, NULL);
+ /* Jump and continue completion for foreign requests, don't do
+ * anything for current request, it will be completed shortly. */
+ if (laiocb->co != qemu_coroutine_self()) {
+ qemu_coroutine_enter(laiocb->co, NULL);
+ }
} else {
laiocb->common.cb(laiocb->common.opaque, ret);
qemu_aio_unref(laiocb);
@@ -312,6 +316,19 @@ static void ioq_submit(LinuxAioState *s)
QSIMPLEQ_SPLIT_AFTER(&s->io_q.pending, aiocb, next, &completed);
} while (ret == len && !QSIMPLEQ_EMPTY(&s->io_q.pending));
s->io_q.blocked = (s->io_q.in_queue > 0);
+
+ if (s->io_q.in_flight) {
+ /* We can try to complete something just right away if there are
+ * still requests in-flight. */
+ qemu_laio_process_completions(s);
+ /*
+ * Even we have completed everything (in_flight == 0), the queue can
+ * have still pended requests (in_queue > 0). We do not attempt to
+ * repeat submission to avoid IO hang. The reason is simple: s->e is
+ * still set and completion callback will be called shortly and all
+ * pended requests will be submitted from there.
+ */
+ }
}
void laio_io_plug(BlockDriverState *bs, LinuxAioState *s)
@@ -370,6 +387,7 @@ int coroutine_fn laio_co_submit(BlockDriverState *bs, LinuxAioState *s, int fd,
.co = qemu_coroutine_self(),
.nbytes = qiov->size,
.ctx = s,
+ .ret = -EINPROGRESS,
.is_read = (type == QEMU_AIO_READ),
.qiov = qiov,
};
@@ -379,7 +397,9 @@ int coroutine_fn laio_co_submit(BlockDriverState *bs, LinuxAioState *s, int fd,
return ret;
}
- qemu_coroutine_yield();
+ if (laiocb.ret == -EINPROGRESS) {
+ qemu_coroutine_yield();
+ }
return laiocb.ret;
}
In order to reduce completion latency it makes sense to harvest completed requests ASAP. Very fast backend device can complete requests just after submission, so it is worth trying to check ring buffer in order to peek completed requests directly after io_submit() has been called. Indeed, this patch reduces the completions latencies and increases the overall throughput, e.g. the following is the percentiles of number of completed requests at once: 1th 10th 20th 30th 40th 50th 60th 70th 80th 90th 99.99th Before 2 4 42 112 128 128 128 128 128 128 128 After 1 1 4 14 33 45 47 48 50 51 108 That means, that before the current patch is applied the ring buffer is observed as full (128 requests were consumed at once) in 60% of calls. After patch is applied the distribution of number of completed requests is "smoother" and the queue (requests in-flight) is almost never full. The fio read results are the following (write results are almost the same and are not showed here): Before ------ job: (groupid=0, jobs=8): err= 0: pid=2227: Tue Jul 19 11:29:50 2016 Description : [Emulation of Storage Server Access Pattern] read : io=54681MB, bw=1822.7MB/s, iops=179779, runt= 30001msec slat (usec): min=172, max=16883, avg=338.35, stdev=109.66 clat (usec): min=1, max=21977, avg=1051.45, stdev=299.29 lat (usec): min=317, max=22521, avg=1389.83, stdev=300.73 clat percentiles (usec): | 1.00th=[ 346], 5.00th=[ 596], 10.00th=[ 708], 20.00th=[ 852], | 30.00th=[ 932], 40.00th=[ 996], 50.00th=[ 1048], 60.00th=[ 1112], | 70.00th=[ 1176], 80.00th=[ 1256], 90.00th=[ 1384], 95.00th=[ 1496], | 99.00th=[ 1800], 99.50th=[ 1928], 99.90th=[ 2320], 99.95th=[ 2672], | 99.99th=[ 4704] bw (KB /s): min=205229, max=553181, per=12.50%, avg=233278.26, stdev=18383.51 After ------ job: (groupid=0, jobs=8): err= 0: pid=2220: Tue Jul 19 11:31:51 2016 Description : [Emulation of Storage Server Access Pattern] read : io=57637MB, bw=1921.2MB/s, iops=189529, runt= 30002msec slat (usec): min=169, max=20636, avg=329.61, stdev=124.18 clat (usec): min=2, max=19592, avg=988.78, stdev=251.04 lat (usec): min=381, max=21067, avg=1318.42, stdev=243.58 clat percentiles (usec): | 1.00th=[ 310], 5.00th=[ 580], 10.00th=[ 748], 20.00th=[ 876], | 30.00th=[ 908], 40.00th=[ 948], 50.00th=[ 1012], 60.00th=[ 1064], | 70.00th=[ 1080], 80.00th=[ 1128], 90.00th=[ 1224], 95.00th=[ 1288], | 99.00th=[ 1496], 99.50th=[ 1608], 99.90th=[ 1960], 99.95th=[ 2256], | 99.99th=[ 5408] bw (KB /s): min=212149, max=390160, per=12.49%, avg=245746.04, stdev=11606.75 Throughput increased from 1822MB/s to 1921MB/s, average completion latencies decreased from 1051us to 988us. Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com> Cc: Stefan Hajnoczi <stefanha@redhat.com> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: qemu-devel@nongnu.org --- block/linux-aio.c | 24 ++++++++++++++++++++++-- 1 file changed, 22 insertions(+), 2 deletions(-)