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From 6f4cb3e1e5d718356f16645e806d47cb2159ae98 Mon Sep 17 00:00:00 2001
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From: Stefano Garzarella <sgarzare@redhat.com>
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Date: Thu, 29 Jul 2021 07:42:33 -0400
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Subject: [PATCH 19/39] linux-aio: limit the batch size using `aio-max-batch`
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parameter
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RH-Author: Miroslav Rezanina <mrezanin@redhat.com>
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RH-MergeRequest: 32: Synchronize with RHEL-AV 8.5 release 27 to RHEL 9
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RH-Commit: [11/15] 44e2f2d294d8ed1d13fb29c5c1599543b86c67e5 (mrezanin/centos-src-qemu-kvm)
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RH-Bugzilla: 1957194
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RH-Acked-by: Stefano Garzarella <sgarzare@redhat.com>
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RH-Acked-by: Kevin Wolf <kwolf@redhat.com>
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RH-Acked-by: Igor Mammedov <imammedo@redhat.com>
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RH-Acked-by: Andrew Jones <drjones@redhat.com>
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When there are multiple queues attached to the same AIO context,
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some requests may experience high latency, since in the worst case
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the AIO engine queue is only flushed when it is full (MAX_EVENTS) or
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there are no more queues plugged.
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Commit 2558cb8dd4 ("linux-aio: increasing MAX_EVENTS to a larger
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hardcoded value") changed MAX_EVENTS from 128 to 1024, to increase
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the number of in-flight requests. But this change also increased
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the potential maximum batch to 1024 elements.
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When there is a single queue attached to the AIO context, the issue
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is mitigated from laio_io_unplug() that will flush the queue every
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time is invoked since there can't be others queue plugged.
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Let's use the new `aio-max-batch` IOThread parameter to mitigate
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this issue, limiting the number of requests in a batch.
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We also define a default value (32): this value is obtained running
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some benchmarks and it represents a good tradeoff between the latency
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increase while a request is queued and the cost of the io_submit(2)
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system call.
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Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
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Message-id: 20210721094211.69853-4-sgarzare@redhat.com
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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(cherry picked from commit d7ddd0a1618a75b31dc308bb37365ce1da972154)
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Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
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Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
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---
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block/linux-aio.c | 9 ++++++++-
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1 file changed, 8 insertions(+), 1 deletion(-)
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diff --git a/block/linux-aio.c b/block/linux-aio.c
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index 3c0527c2bf..0dab507b71 100644
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--- a/block/linux-aio.c
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+++ b/block/linux-aio.c
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@@ -28,6 +28,9 @@
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*/
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#define MAX_EVENTS 1024
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+/* Maximum number of requests in a batch. (default value) */
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+#define DEFAULT_MAX_BATCH 32
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+
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struct qemu_laiocb {
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Coroutine *co;
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LinuxAioState *ctx;
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@@ -351,6 +354,10 @@ static int laio_do_submit(int fd, struct qemu_laiocb *laiocb, off_t offset,
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LinuxAioState *s = laiocb->ctx;
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struct iocb *iocbs = &laiocb->iocb;
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QEMUIOVector *qiov = laiocb->qiov;
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+ int64_t max_batch = s->aio_context->aio_max_batch ?: DEFAULT_MAX_BATCH;
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+
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+ /* limit the batch with the number of available events */
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+ max_batch = MIN_NON_ZERO(MAX_EVENTS - s->io_q.in_flight, max_batch);
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switch (type) {
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case QEMU_AIO_WRITE:
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@@ -371,7 +378,7 @@ static int laio_do_submit(int fd, struct qemu_laiocb *laiocb, off_t offset,
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s->io_q.in_queue++;
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if (!s->io_q.blocked &&
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(!s->io_q.plugged ||
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- s->io_q.in_flight + s->io_q.in_queue >= MAX_EVENTS)) {
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+ s->io_q.in_queue >= max_batch)) {
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ioq_submit(s);
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}
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--
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2.27.0
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