@@ -28,6 +28,9 @@
*/
#define MAX_EVENTS 1024
+/* Maximum number of requests in a batch. (default value) */
+#define DEFAULT_MAX_BATCH 32
+
struct qemu_laiocb {
Coroutine *co;
LinuxAioState *ctx;
@@ -351,6 +354,10 @@ static int laio_do_submit(int fd, struct qemu_laiocb *laiocb, off_t offset,
LinuxAioState *s = laiocb->ctx;
struct iocb *iocbs = &laiocb->iocb;
QEMUIOVector *qiov = laiocb->qiov;
+ int64_t max_batch = s->aio_context->aio_max_batch ?: DEFAULT_MAX_BATCH;
+
+ /* limit the batch with the number of available events */
+ max_batch = MIN_NON_ZERO(MAX_EVENTS - s->io_q.in_flight, max_batch);
switch (type) {
case QEMU_AIO_WRITE:
@@ -371,7 +378,7 @@ static int laio_do_submit(int fd, struct qemu_laiocb *laiocb, off_t offset,
s->io_q.in_queue++;
if (!s->io_q.blocked &&
(!s->io_q.plugged ||
- s->io_q.in_flight + s->io_q.in_queue >= MAX_EVENTS)) {
+ s->io_q.in_queue >= max_batch)) {
ioq_submit(s);
}
When there are multiple queues attached to the same AIO context, some requests may experience high latency, since in the worst case the AIO engine queue is only flushed when it is full (MAX_EVENTS) or there are no more queues plugged. Commit 2558cb8dd4 ("linux-aio: increasing MAX_EVENTS to a larger hardcoded value") changed MAX_EVENTS from 128 to 1024, to increase the number of in-flight requests. But this change also increased the potential maximum batch to 1024 elements. When there is a single queue attached to the AIO context, the issue is mitigated from laio_io_unplug() that will flush the queue every time is invoked since there can't be others queue plugged. Let's use the new `aio-max-batch` IOThread parameter to mitigate this issue, limiting the number of requests in a batch. We also define a default value (32): this value is obtained running some benchmarks and it represents a good tradeoff between the latency increase while a request is queued and the cost of the io_submit(2) system call. Signed-off-by: Stefano Garzarella <sgarzare@redhat.com> --- Notes: v2: - limit the batch with the number of available events [stefanha] block/linux-aio.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)