From 6f4cb3e1e5d718356f16645e806d47cb2159ae98 Mon Sep 17 00:00:00 2001
From: Stefano Garzarella <sgarzare@redhat.com>
Date: Thu, 29 Jul 2021 07:42:33 -0400
Subject: [PATCH 19/39] linux-aio: limit the batch size using `aio-max-batch`
 parameter

RH-Author: Miroslav Rezanina <mrezanin@redhat.com>
RH-MergeRequest: 32: Synchronize with RHEL-AV 8.5 release 27 to RHEL 9
RH-Commit: [11/15] 44e2f2d294d8ed1d13fb29c5c1599543b86c67e5 (mrezanin/centos-src-qemu-kvm)
RH-Bugzilla: 1957194
RH-Acked-by: Stefano Garzarella <sgarzare@redhat.com>
RH-Acked-by: Kevin Wolf <kwolf@redhat.com>
RH-Acked-by: Igor Mammedov <imammedo@redhat.com>
RH-Acked-by: Andrew Jones <drjones@redhat.com>

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>
Message-id: 20210721094211.69853-4-sgarzare@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit d7ddd0a1618a75b31dc308bb37365ce1da972154)
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
 block/linux-aio.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/block/linux-aio.c b/block/linux-aio.c
index 3c0527c2bf..0dab507b71 100644
--- a/block/linux-aio.c
+++ b/block/linux-aio.c
@@ -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);
     }
 
-- 
2.27.0