From: Stefano Garzarella <sgarzare@redhat.com>

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>

Message-id: 20210721094211.69853-3-sgarzare@redhat.com

Signed-off-by: Stefan Hajnoczi <stefanha@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 XXXXXXX..XXXXXXX 100644

--- a/qapi/misc.json

+++ b/qapi/misc.json

@@ -XXX,XX +XXX,XX @@

# @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 batch for the AIO engine,

+# 0 means that the engine will use its default (since 6.1)

+#

# Since: 2.0

##

{ 'struct': 'IOThreadInfo',

@@ -XXX,XX +XXX,XX @@

'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 XXXXXXX..XXXXXXX 100644

--- a/qapi/qom.json

+++ b/qapi/qom.json

@@ -XXX,XX +XXX,XX @@

# 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 batch 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 XXXXXXX..XXXXXXX 100644

--- a/include/block/aio.h

+++ b/include/block/aio.h

@@ -XXX,XX +XXX,XX @@ 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

@@ -XXX,XX +XXX,XX @@ 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 batch, 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 XXXXXXX..XXXXXXX 100644

--- a/include/sysemu/iothread.h

+++ b/include/sysemu/iothread.h

@@ -XXX,XX +XXX,XX @@ 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 XXXXXXX..XXXXXXX 100644

--- a/iothread.c

+++ b/iothread.c

@@ -XXX,XX +XXX,XX @@ 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;

@@ -XXX,XX +XXX,XX @@ 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);

@@ -XXX,XX +XXX,XX @@ 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)

@@ -XXX,XX +XXX,XX @@ 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);

@@ -XXX,XX +XXX,XX @@ 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 = {

@@ -XXX,XX +XXX,XX @@ 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 XXXXXXX..XXXXXXX 100644

--- a/monitor/hmp-cmds.c

+++ b/monitor/hmp-cmds.c

@@ -XXX,XX +XXX,XX @@ 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 XXXXXXX..XXXXXXX 100644

--- a/util/aio-posix.c

+++ b/util/aio-posix.c

@@ -XXX,XX +XXX,XX @@ 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 XXXXXXX..XXXXXXX 100644

--- a/util/aio-win32.c

+++ b/util/aio-win32.c

@@ -XXX,XX +XXX,XX @@ 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 XXXXXXX..XXXXXXX 100644

--- a/util/async.c

+++ b/util/async.c

@@ -XXX,XX +XXX,XX @@ 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 XXXXXXX..XXXXXXX 100644

--- a/qemu-options.hx

+++ b/qemu-options.hx

@@ -XXX,XX +XXX,XX @@ 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.

@@ -XXX,XX +XXX,XX @@ 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

From: Stefano Garzarella <sgarzare@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>

---

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 XXXXXXX..XXXXXXX 100644

--- a/block/linux-aio.c

+++ b/block/linux-aio.c

@@ -XXX,XX +XXX,XX @@

*/

#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;

@@ -XXX,XX +XXX,XX @@ 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:

@@ -XXX,XX +XXX,XX @@ 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