[PATCH 0/3] linux-aio: limit the batch size to reduce queue latency

[PATCH 0/3] linux-aio: limit the batch size to reduce queue latency

[Stefano Garzarella]

This series add a new `aio-max-batch` parameter to IOThread, and use it in the
Linux AIO backend to limit the batch size (number of request submitted to the
kernel through io_submit(2)).

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.

The problem is noticeable when we have a lot of requests in flight and multiple
queues attached to the same AIO context.
In this case we potentially create very large batches. Instead, when we have
a single queue, the batch is limited because when the queue is unplugged,
there is a call to io_submit(2).
In practice, io_submit(2) was called only when there are no more queues plugged
in or when we fill the AIO queue (MAX_EVENTS = 1024).

I run some benchmarks to choose 32 as default batch value for Linux AIO.
Below the kIOPS measured with fio running in the guest (average over 3 runs):

                   |   master  |           with this series applied            |
                   |687f9f7834e| maxbatch=8|maxbatch=16|maxbatch=32|maxbatch=64|
          # queues | 1q  | 4qs | 1q  | 4qs | 1q  | 4qs | 1q  | 4qs | 1q  | 4qs |
-- randread tests -|-----------------------------------------------------------|
bs=4k iodepth=1    | 193 | 188 | 204 | 198 | 194 | 202 | 201 | 213 | 195 | 201 |
bs=4k iodepth=8    | 241 | 265 | 247 | 248 | 249 | 250 | 257 | 269 | 270 | 240 |
bs=4k iodepth=64   | 216 | 202 | 257 | 269 | 269 | 256 | 258 | 271 | 254 | 251 |
bs=4k iodepth=128  | 212 | 177 | 267 | 253 | 285 | 271 | 245 | 281 | 255 | 269 |
bs=16k iodepth=1   | 130 | 133 | 137 | 137 | 130 | 130 | 130 | 130 | 130 | 130 |
bs=16k iodepth=8   | 130 | 137 | 144 | 137 | 131 | 130 | 131 | 131 | 130 | 131 |
bs=16k iodepth=64  | 130 | 104 | 137 | 134 | 131 | 128 | 131 | 128 | 137 | 128 |
bs=16k iodepth=128 | 130 | 101 | 137 | 134 | 131 | 129 | 131 | 129 | 138 | 129 |

1q  = virtio-blk device with a single queue
4qs = virito-blk device with multi queues (one queue per vCPU - 4)

I reported only the most significant tests, but I also did other tests to
make sure there were no regressions, here the full report:
https://docs.google.com/spreadsheets/d/11X3_5FJu7pnMTlf4ZatRDvsnU9K3EPj6Mn3aJIsE4tI

Test environment:
- Disk: Intel Corporation NVMe Datacenter SSD [Optane]
- CPU: Intel(R) Xeon(R) Silver 4214 CPU @ 2.20GHz
- QEMU: qemu-system-x86_64 -machine q35,accel=kvm -smp 4 -m 4096 \
          ... \
          -object iothread,id=iothread0,aio-max-batch=${MAX_BATCH} \
          -device virtio-blk-pci,iothread=iothread0,num-queues=${NUM_QUEUES}

- benchmark: fio --ioengine=libaio --thread --group_reporting \
                 --number_ios=200000 --direct=1 --filename=/dev/vdb \
                 --rw=${TEST} --bs=${BS} --iodepth=${IODEPTH} --numjobs=16

Next steps:
 - benchmark io_uring and use `aio-max-batch` also there
 - make MAX_EVENTS parametric adding a new `aio-max-events` parameter

Comments and suggestions are welcome :-)

Thanks,
Stefano

Stefano Garzarella (3):
  iothread: generalize iothread_set_param/iothread_get_param
  iothread: add aio-max-batch parameter
  linux-aio: limit the batch size using `aio-max-batch` parameter

 qapi/misc.json            |  6 ++-
 qapi/qom.json             |  7 +++-
 include/block/aio.h       | 12 ++++++
 include/sysemu/iothread.h |  3 ++
 block/linux-aio.c         |  6 ++-
 iothread.c                | 82 ++++++++++++++++++++++++++++++++++-----
 monitor/hmp-cmds.c        |  2 +
 util/aio-posix.c          | 12 ++++++
 util/aio-win32.c          |  5 +++
 util/async.c              |  2 +
 qemu-options.hx           |  8 +++-
 11 files changed, 131 insertions(+), 14 deletions(-)

-- 
2.31.1

[PATCH 1/3] iothread: generalize iothread_set_param/iothread_get_param

[Stefano Garzarella]

Changes in preparation for next patches where we add a new
parameter not related to the poll mechanism.

Let's add two new generic functions (iothread_set_param and
iothread_get_param) that we use to set and get IOThread
parameters.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
 iothread.c | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/iothread.c b/iothread.c
index 2c5ccd7367..103679a16b 100644
--- a/iothread.c
+++ b/iothread.c
@@ -213,7 +213,7 @@ static PollParamInfo poll_shrink_info = {
     "poll-shrink", offsetof(IOThread, poll_shrink),
 };
 
-static void iothread_get_poll_param(Object *obj, Visitor *v,
+static void iothread_get_param(Object *obj, Visitor *v,
         const char *name, void *opaque, Error **errp)
 {
     IOThread *iothread = IOTHREAD(obj);
@@ -223,7 +223,7 @@ static void iothread_get_poll_param(Object *obj, Visitor *v,
     visit_type_int64(v, name, field, errp);
 }
 
-static void iothread_set_poll_param(Object *obj, Visitor *v,
+static bool iothread_set_param(Object *obj, Visitor *v,
         const char *name, void *opaque, Error **errp)
 {
     IOThread *iothread = IOTHREAD(obj);
@@ -232,17 +232,36 @@ static void iothread_set_poll_param(Object *obj, Visitor *v,
     int64_t value;
 
     if (!visit_type_int64(v, name, &value, errp)) {
-        return;
+        return false;
     }
 
     if (value < 0) {
         error_setg(errp, "%s value must be in range [0, %" PRId64 "]",
                    info->name, INT64_MAX);
-        return;
+        return false;
     }
 
     *field = value;
 
+    return true;
+}
+
+static void iothread_get_poll_param(Object *obj, Visitor *v,
+        const char *name, void *opaque, Error **errp)
+{
+
+    iothread_get_param(obj, v, name, opaque, errp);
+}
+
+static void iothread_set_poll_param(Object *obj, Visitor *v,
+        const char *name, void *opaque, Error **errp)
+{
+    IOThread *iothread = IOTHREAD(obj);
+
+    if (!iothread_set_param(obj, v, name, opaque, errp)) {
+        return;
+    }
+
     if (iothread->ctx) {
         aio_context_set_poll_params(iothread->ctx,
                                     iothread->poll_max_ns,
-- 
2.31.1

[PATCH 2/3] iothread: add aio-max-batch parameter

[Stefano Garzarella]

The `aio-max-batch` parameter will be propagated to AIO engines
and it will be used to control the maximum number of queued requests.

When there are in queue a number of requests equal to `aio-max-batch`,
the engine invokes the system call to forward the requests to the kernel.

This parameter allows us to control the maximum batch size to reduce
the latency that requests might accumulate while queued in the AIO
engine queue.

If `aio-max-batch` is equal to 0 (default value), the AIO engine will
use its default maximum batch size value.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
 qapi/misc.json            |  6 ++++-
 qapi/qom.json             |  7 ++++-
 include/block/aio.h       | 12 +++++++++
 include/sysemu/iothread.h |  3 +++
 iothread.c                | 55 +++++++++++++++++++++++++++++++++++----
 monitor/hmp-cmds.c        |  2 ++
 util/aio-posix.c          | 12 +++++++++
 util/aio-win32.c          |  5 ++++
 util/async.c              |  2 ++
 qemu-options.hx           |  8 ++++--
 10 files changed, 103 insertions(+), 9 deletions(-)

diff --git a/qapi/misc.json b/qapi/misc.json
index 156f98203e..f64bb69f74 100644
--- a/qapi/misc.json
+++ b/qapi/misc.json
@@ -86,6 +86,9 @@
 # @poll-shrink: how many ns will be removed from polling time, 0 means that
 #               it's not configured (since 2.9)
 #
+# @aio-max-batch: maximum number of requests in a bacth for the AIO engine,
+#                 0 means that the engine will use its default (since 6.1)
+#
 # Since: 2.0
 ##
 { 'struct': 'IOThreadInfo',
@@ -93,7 +96,8 @@
            'thread-id': 'int',
            'poll-max-ns': 'int',
            'poll-grow': 'int',
-           'poll-shrink': 'int' } }
+           'poll-shrink': 'int',
+           'aio-max-batch': 'int' } }
 
 ##
 # @query-iothreads:
diff --git a/qapi/qom.json b/qapi/qom.json
index 652be317b8..23fd586614 100644
--- a/qapi/qom.json
+++ b/qapi/qom.json
@@ -516,12 +516,17 @@
 #               algorithm detects it is spending too long polling without
 #               encountering events. 0 selects a default behaviour (default: 0)
 #
+# @aio-max-batch: maximum number of requests in a bacth for the AIO engine,
+#                 0 means that the engine will use its default
+#                 (default:0, since 6.1)
+#
 # Since: 2.0
 ##
 { 'struct': 'IothreadProperties',
   'data': { '*poll-max-ns': 'int',
             '*poll-grow': 'int',
-            '*poll-shrink': 'int' } }
+            '*poll-shrink': 'int',
+            '*aio-max-batch': 'int' } }
 
 ##
 # @MemoryBackendProperties:
diff --git a/include/block/aio.h b/include/block/aio.h
index 10fcae1515..5b7983348f 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -232,6 +232,9 @@ struct AioContext {
     int64_t poll_grow;      /* polling time growth factor */
     int64_t poll_shrink;    /* polling time shrink factor */
 
+    /* AIO engine parameters */
+    int64_t aio_max_batch;  /* maximum number of requests in a batch */
+
     /*
      * List of handlers participating in userspace polling.  Protected by
      * ctx->list_lock.  Iterated and modified mostly by the event loop thread
@@ -730,4 +733,13 @@ void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
                                  int64_t grow, int64_t shrink,
                                  Error **errp);
 
+/**
+ * aio_context_set_aio_params:
+ * @ctx: the aio context
+ * @max_batch: maximum number of requests in a bacth, 0 means that the
+ *             engine will use its default
+ */
+void aio_context_set_aio_params(AioContext *ctx, int64_t max_batch,
+                                Error **errp);
+
 #endif
diff --git a/include/sysemu/iothread.h b/include/sysemu/iothread.h
index f177142f16..7f714bd136 100644
--- a/include/sysemu/iothread.h
+++ b/include/sysemu/iothread.h
@@ -37,6 +37,9 @@ struct IOThread {
     int64_t poll_max_ns;
     int64_t poll_grow;
     int64_t poll_shrink;
+
+    /* AioContext AIO engine parameters */
+    int64_t aio_max_batch;
 };
 typedef struct IOThread IOThread;
 
diff --git a/iothread.c b/iothread.c
index 103679a16b..ddbbde61f7 100644
--- a/iothread.c
+++ b/iothread.c
@@ -152,6 +152,24 @@ static void iothread_init_gcontext(IOThread *iothread)
     iothread->main_loop = g_main_loop_new(iothread->worker_context, TRUE);
 }
 
+static void iothread_set_aio_context_params(IOThread *iothread, Error **errp)
+{
+    ERRP_GUARD();
+
+    aio_context_set_poll_params(iothread->ctx,
+                                iothread->poll_max_ns,
+                                iothread->poll_grow,
+                                iothread->poll_shrink,
+                                errp);
+    if (*errp) {
+        return;
+    }
+
+    aio_context_set_aio_params(iothread->ctx,
+                               iothread->aio_max_batch,
+                               errp);
+}
+
 static void iothread_complete(UserCreatable *obj, Error **errp)
 {
     Error *local_error = NULL;
@@ -171,11 +189,7 @@ static void iothread_complete(UserCreatable *obj, Error **errp)
      */
     iothread_init_gcontext(iothread);
 
-    aio_context_set_poll_params(iothread->ctx,
-                                iothread->poll_max_ns,
-                                iothread->poll_grow,
-                                iothread->poll_shrink,
-                                &local_error);
+    iothread_set_aio_context_params(iothread, &local_error);
     if (local_error) {
         error_propagate(errp, local_error);
         aio_context_unref(iothread->ctx);
@@ -212,6 +226,9 @@ static PollParamInfo poll_grow_info = {
 static PollParamInfo poll_shrink_info = {
     "poll-shrink", offsetof(IOThread, poll_shrink),
 };
+static PollParamInfo aio_max_batch_info = {
+    "aio-max-batch", offsetof(IOThread, aio_max_batch),
+};
 
 static void iothread_get_param(Object *obj, Visitor *v,
         const char *name, void *opaque, Error **errp)
@@ -271,6 +288,29 @@ static void iothread_set_poll_param(Object *obj, Visitor *v,
     }
 }
 
+static void iothread_get_aio_param(Object *obj, Visitor *v,
+        const char *name, void *opaque, Error **errp)
+{
+
+    iothread_get_param(obj, v, name, opaque, errp);
+}
+
+static void iothread_set_aio_param(Object *obj, Visitor *v,
+        const char *name, void *opaque, Error **errp)
+{
+    IOThread *iothread = IOTHREAD(obj);
+
+    if (!iothread_set_param(obj, v, name, opaque, errp)) {
+        return;
+    }
+
+    if (iothread->ctx) {
+        aio_context_set_aio_params(iothread->ctx,
+                                   iothread->aio_max_batch,
+                                   errp);
+    }
+}
+
 static void iothread_class_init(ObjectClass *klass, void *class_data)
 {
     UserCreatableClass *ucc = USER_CREATABLE_CLASS(klass);
@@ -288,6 +328,10 @@ static void iothread_class_init(ObjectClass *klass, void *class_data)
                               iothread_get_poll_param,
                               iothread_set_poll_param,
                               NULL, &poll_shrink_info);
+    object_class_property_add(klass, "aio-max-batch", "int",
+                              iothread_get_aio_param,
+                              iothread_set_aio_param,
+                              NULL, &aio_max_batch_info);
 }
 
 static const TypeInfo iothread_info = {
@@ -337,6 +381,7 @@ static int query_one_iothread(Object *object, void *opaque)
     info->poll_max_ns = iothread->poll_max_ns;
     info->poll_grow = iothread->poll_grow;
     info->poll_shrink = iothread->poll_shrink;
+    info->aio_max_batch = iothread->aio_max_batch;
 
     QAPI_LIST_APPEND(*tail, info);
     return 0;
diff --git a/monitor/hmp-cmds.c b/monitor/hmp-cmds.c
index 0942027208..e00255f7ee 100644
--- a/monitor/hmp-cmds.c
+++ b/monitor/hmp-cmds.c
@@ -1893,6 +1893,8 @@ void hmp_info_iothreads(Monitor *mon, const QDict *qdict)
         monitor_printf(mon, "  poll-max-ns=%" PRId64 "\n", value->poll_max_ns);
         monitor_printf(mon, "  poll-grow=%" PRId64 "\n", value->poll_grow);
         monitor_printf(mon, "  poll-shrink=%" PRId64 "\n", value->poll_shrink);
+        monitor_printf(mon, "  aio-max-batch=%" PRId64 "\n",
+                       value->aio_max_batch);
     }
 
     qapi_free_IOThreadInfoList(info_list);
diff --git a/util/aio-posix.c b/util/aio-posix.c
index 30f5354b1e..2b86777e91 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -716,3 +716,15 @@ void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
 
     aio_notify(ctx);
 }
+
+void aio_context_set_aio_params(AioContext *ctx, int64_t max_batch,
+                                Error **errp)
+{
+    /*
+     * No thread synchronization here, it doesn't matter if an incorrect value
+     * is used once.
+     */
+    ctx->aio_max_batch = max_batch;
+
+    aio_notify(ctx);
+}
diff --git a/util/aio-win32.c b/util/aio-win32.c
index 168717b51b..d5b09a1193 100644
--- a/util/aio-win32.c
+++ b/util/aio-win32.c
@@ -440,3 +440,8 @@ void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
         error_setg(errp, "AioContext polling is not implemented on Windows");
     }
 }
+
+void aio_context_set_aio_params(AioContext *ctx, int64_t max_batch,
+                                Error **errp)
+{
+}
diff --git a/util/async.c b/util/async.c
index 5d9b7cc1eb..f3510ea4f9 100644
--- a/util/async.c
+++ b/util/async.c
@@ -537,6 +537,8 @@ AioContext *aio_context_new(Error **errp)
     ctx->poll_grow = 0;
     ctx->poll_shrink = 0;
 
+    ctx->aio_max_batch = 0;
+
     return ctx;
 fail:
     g_source_destroy(&ctx->source);
diff --git a/qemu-options.hx b/qemu-options.hx
index 8965dabc83..43c03a8e3c 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -5255,7 +5255,7 @@ SRST
 
             CN=laptop.example.com,O=Example Home,L=London,ST=London,C=GB
 
-    ``-object iothread,id=id,poll-max-ns=poll-max-ns,poll-grow=poll-grow,poll-shrink=poll-shrink``
+    ``-object iothread,id=id,poll-max-ns=poll-max-ns,poll-grow=poll-grow,poll-shrink=poll-shrink,aio-max-batch=aio-max-batch``
         Creates a dedicated event loop thread that devices can be
         assigned to. This is known as an IOThread. By default device
         emulation happens in vCPU threads or the main event loop thread.
@@ -5291,7 +5291,11 @@ SRST
         the polling time when the algorithm detects it is spending too
         long polling without encountering events.
 
-        The polling parameters can be modified at run-time using the
+        The ``aio-max-batch`` parameter is the maximum number of requests
+        in a batch for the AIO engine, 0 means that the engine will use
+        its default.
+
+        The IOThread parameters can be modified at run-time using the
         ``qom-set`` command (where ``iothread1`` is the IOThread's
         ``id``):
 
-- 
2.31.1

[PATCH 3/3] linux-aio: limit the batch size using `aio-max-batch` parameter

[Stefano Garzarella]

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>
---
 block/linux-aio.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/block/linux-aio.c b/block/linux-aio.c
index 3c0527c2bf..8a7bb136fc 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 +354,7 @@ 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;
 
     switch (type) {
     case QEMU_AIO_WRITE:
@@ -371,7 +375,7 @@ 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);
     }
 
-- 
2.31.1

Re: [PATCH 2/3] iothread: add aio-max-batch parameter

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 1309 bytes --]

On Wed, Jul 07, 2021 at 05:00:18PM +0200, Stefano Garzarella wrote:
> diff --git a/qapi/misc.json b/qapi/misc.json
> index 156f98203e..f64bb69f74 100644
> --- a/qapi/misc.json
> +++ b/qapi/misc.json
> @@ -86,6 +86,9 @@
>  # @poll-shrink: how many ns will be removed from polling time, 0 means that
>  #               it's not configured (since 2.9)
>  #
> +# @aio-max-batch: maximum number of requests in a bacth for the AIO engine,

s/bacth/batch/

> +#                 0 means that the engine will use its default (since 6.1)
> +#
>  # Since: 2.0
>  ##
>  { 'struct': 'IOThreadInfo',
> @@ -93,7 +96,8 @@
>             'thread-id': 'int',
>             'poll-max-ns': 'int',
>             'poll-grow': 'int',
> -           'poll-shrink': 'int' } }
> +           'poll-shrink': 'int',
> +           'aio-max-batch': 'int' } }
>  
>  ##
>  # @query-iothreads:
> diff --git a/qapi/qom.json b/qapi/qom.json
> index 652be317b8..23fd586614 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -516,12 +516,17 @@
>  #               algorithm detects it is spending too long polling without
>  #               encountering events. 0 selects a default behaviour (default: 0)
>  #
> +# @aio-max-batch: maximum number of requests in a bacth for the AIO engine,

s/bacth/batch/

[-- Attachment #2: signature.asc --]
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Re: [PATCH 3/3] linux-aio: limit the batch size using `aio-max-batch` parameter

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 719 bytes --]

On Wed, Jul 07, 2021 at 05:00:19PM +0200, Stefano Garzarella wrote:
> @@ -371,7 +375,7 @@ 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)) {

Is it safe to drop the MAX_EVENTS case?

Perhaps the following can be used:

  int64_t max_batch = s->aio_context->aio_max_batch ?: DEFAULT_MAX_BATCH;
  max_batch = MIN_NON_ZERO(MAX_EVENTS - s->io_q.in_flight + s->io_q.in_queue, max_batch);

Here we'll only need to check against max_batch but it takes into
account MAX_EVENT and in_flight.

Stefan

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Re: [PATCH 2/3] iothread: add aio-max-batch parameter

[Stefano Garzarella]

On Tue, Jul 13, 2021 at 03:51:15PM +0100, Stefan Hajnoczi wrote:
>On Wed, Jul 07, 2021 at 05:00:18PM +0200, Stefano Garzarella wrote:
>> diff --git a/qapi/misc.json b/qapi/misc.json
>> index 156f98203e..f64bb69f74 100644
>> --- a/qapi/misc.json
>> +++ b/qapi/misc.json
>> @@ -86,6 +86,9 @@
>>  # @poll-shrink: how many ns will be removed from polling time, 0 means that
>>  #               it's not configured (since 2.9)
>>  #
>> +# @aio-max-batch: maximum number of requests in a bacth for the AIO engine,
>
>s/bacth/batch/
>
>> +#                 0 means that the engine will use its default (since 6.1)
>> +#
>>  # Since: 2.0
>>  ##
>>  { 'struct': 'IOThreadInfo',
>> @@ -93,7 +96,8 @@
>>             'thread-id': 'int',
>>             'poll-max-ns': 'int',
>>             'poll-grow': 'int',
>> -           'poll-shrink': 'int' } }
>> +           'poll-shrink': 'int',
>> +           'aio-max-batch': 'int' } }
>>
>>  ##
>>  # @query-iothreads:
>> diff --git a/qapi/qom.json b/qapi/qom.json
>> index 652be317b8..23fd586614 100644
>> --- a/qapi/qom.json
>> +++ b/qapi/qom.json
>> @@ -516,12 +516,17 @@
>>  #               algorithm detects it is spending too long polling without
>>  #               encountering events. 0 selects a default behaviour (default: 0)
>>  #
>> +# @aio-max-batch: maximum number of requests in a bacth for the AIO engine,
>
>s/bacth/batch/

Fixed in v2 also in include/block/aio.h :-)

Thanks,
Stefano

Re: [PATCH 3/3] linux-aio: limit the batch size using `aio-max-batch` parameter

[Stefano Garzarella]

On Tue, Jul 13, 2021 at 03:58:04PM +0100, Stefan Hajnoczi wrote:
>On Wed, Jul 07, 2021 at 05:00:19PM +0200, Stefano Garzarella wrote:
>> @@ -371,7 +375,7 @@ 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)) {
>
>Is it safe to drop the MAX_EVENTS case?

I think it is safe since in ioq_submit() we have this check while 
dequeueing:

         QSIMPLEQ_FOREACH(aiocb, &s->io_q.pending, next) {
             iocbs[len++] = &aiocb->iocb;
             if (s->io_q.in_flight + len >= MAX_EVENTS) {
                 break;
             }
         }

But in term of performance, I think is better what you're suggesting, 
because if we have fewer slots available than `max_batch`, here we were 
delaying the call to io_submit().

>
>Perhaps the following can be used:
>
>  int64_t max_batch = s->aio_context->aio_max_batch ?: DEFAULT_MAX_BATCH;
>  max_batch = MIN_NON_ZERO(MAX_EVENTS - s->io_q.in_flight + s->io_q.in_queue, max_batch);
>

Since we will compare `in_queue` with `max_batch`, should we remove it 
from this expression?

I mean:

   int64_t max_batch = s->aio_context->aio_max_batch ?: DEFAULT_MAX_BATCH;
   max_batch = MIN_NON_ZERO(MAX_EVENTS - s->io_q.in_flight, max_batch);

then as it is in this patch:

   s->io_q.in_queue++;
   if (!s->io_q.blocked &&
       (!s->io_q.plugged ||
        s->io_q.in_queue >= max_batch)) {
       ioq_submit(s);
   }

>Here we'll only need to check against max_batch but it takes into
>account MAX_EVENT and in_flight.

Thanks,
Stefano

Re: [PATCH 3/3] linux-aio: limit the batch size using `aio-max-batch` parameter

[Stefan Hajnoczi]

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On Mon, Jul 19, 2021 at 12:35:53PM +0200, Stefano Garzarella wrote:
> On Tue, Jul 13, 2021 at 03:58:04PM +0100, Stefan Hajnoczi wrote:
> > On Wed, Jul 07, 2021 at 05:00:19PM +0200, Stefano Garzarella wrote:
> > > @@ -371,7 +375,7 @@ 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)) {
> > 
> > Is it safe to drop the MAX_EVENTS case?
> 
> I think it is safe since in ioq_submit() we have this check while
> dequeueing:
> 
>         QSIMPLEQ_FOREACH(aiocb, &s->io_q.pending, next) {
>             iocbs[len++] = &aiocb->iocb;
>             if (s->io_q.in_flight + len >= MAX_EVENTS) {
>                 break;
>             }
>         }
> 
> But in term of performance, I think is better what you're suggesting,
> because if we have fewer slots available than `max_batch`, here we were
> delaying the call to io_submit().
> 
> > 
> > Perhaps the following can be used:
> > 
> >  int64_t max_batch = s->aio_context->aio_max_batch ?: DEFAULT_MAX_BATCH;
> >  max_batch = MIN_NON_ZERO(MAX_EVENTS - s->io_q.in_flight + s->io_q.in_queue, max_batch);
> > 
> 
> Since we will compare `in_queue` with `max_batch`, should we remove it from
> this expression?
> 
> I mean:
> 
>   int64_t max_batch = s->aio_context->aio_max_batch ?: DEFAULT_MAX_BATCH;
>   max_batch = MIN_NON_ZERO(MAX_EVENTS - s->io_q.in_flight, max_batch);
> 
> then as it is in this patch:
> 
>   s->io_q.in_queue++;
>   if (!s->io_q.blocked &&
>       (!s->io_q.plugged ||
>        s->io_q.in_queue >= max_batch)) {
>       ioq_submit(s);
>   }

Good.

Stefan

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