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qapi/iothread: introduce poll-weight parameter for aio-poll

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Introduce a configurable poll-weight parameter for adaptive polling
in IOThread. This parameter replaces the hardcoded POLL_WEIGHT_SHIFT
constant, allowing runtime control over how much the most recent
event interval affects the next polling duration calculation.

The poll-weight parameter uses a shift value where larger values
decrease the weight of the current interval, enabling more gradual
adjustments. When set to 0, a default value of 3 is used (meaning
the current interval contributes approximately 1/8 to the weighted
average).

This patch also removes the hardcoded default value checks from
adjust_polling_time(). Instead, poll-grow, poll-shrink, and
poll-weight now use default values initialized in iothread.c
during IOThread creation.

Signed-off-by: Jaehoon Kim <jhkim@linux.ibm.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20260423195918.661299-4-jhkim@linux.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Jaehoon Kim
2026-04-23 14:59:18 -05:00
committed by Stefan Hajnoczi
parent 9563d0b5e2
commit 9fea80ddf0
11 changed files with 95 additions and 42 deletions
Download Patch File Download Diff File Expand all files Collapse all files

4
include/qemu/aio.h
View File

@@ -310,6 +310,7 @@ struct AioContext {
int64_t poll_max_ns; /* maximum polling time in nanoseconds */
int64_t poll_grow; /* polling time growth factor */
int64_t poll_shrink; /* polling time shrink factor */
int64_t poll_weight; /* weight of current interval in calculation */
/* AIO engine parameters */
int64_t aio_max_batch; /* maximum number of requests in a batch */
@@ -791,12 +792,13 @@ void aio_context_destroy(AioContext *ctx);
* @max_ns: how long to busy poll for, in nanoseconds
* @grow: polling time growth factor
* @shrink: polling time shrink factor
* @weight: weight factor applied to the current polling interval
*
* Poll mode can be disabled by setting poll_max_ns to 0.
*/
void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
int64_t grow, int64_t shrink,
Error **errp);
int64_t weight, Error **errp);
/**
* aio_context_set_aio_params:

18
include/system/iothread.h
View File

@@ -21,6 +21,23 @@
#define TYPE_IOTHREAD "iothread"
#ifdef CONFIG_POSIX
/*
* Benchmark results from 2016 on NVMe SSD drives show max polling times around
* 16-32 microseconds yield IOPS improvements for both iodepth=1 and iodepth=32
* workloads.
*/
#define IOTHREAD_POLL_MAX_NS_DEFAULT 32768ULL
#define IOTHREAD_POLL_GROW_DEFAULT 2ULL
#define IOTHREAD_POLL_SHRINK_DEFAULT 2ULL
#define IOTHREAD_POLL_WEIGHT_DEFAULT 3ULL
#else
#define IOTHREAD_POLL_MAX_NS_DEFAULT 0ULL
#define IOTHREAD_POLL_GROW_DEFAULT 0ULL
#define IOTHREAD_POLL_SHRINK_DEFAULT 0ULL
#define IOTHREAD_POLL_WEIGHT_DEFAULT 0ULL
#endif
struct IOThread {
EventLoopBase parent_obj;
@@ -38,6 +55,7 @@ struct IOThread {
int64_t poll_max_ns;
int64_t poll_grow;
int64_t poll_shrink;
int64_t poll_weight;
};
typedef struct IOThread IOThread;

45
iothread.c
View File

@@ -25,17 +25,6 @@
#include "qemu/rcu.h"
#include "qemu/main-loop.h"
#ifdef CONFIG_POSIX
/* Benchmark results from 2016 on NVMe SSD drives show max polling times around
* 16-32 microseconds yield IOPS improvements for both iodepth=1 and iodepth=32
* workloads.
*/
#define IOTHREAD_POLL_MAX_NS_DEFAULT 32768ULL
#else
#define IOTHREAD_POLL_MAX_NS_DEFAULT 0ULL
#endif
static void *iothread_run(void *opaque)
{
IOThread *iothread = opaque;
@@ -103,6 +92,10 @@ static void iothread_instance_init(Object *obj)
IOThread *iothread = IOTHREAD(obj);
iothread->poll_max_ns = IOTHREAD_POLL_MAX_NS_DEFAULT;
iothread->poll_grow = IOTHREAD_POLL_GROW_DEFAULT;
iothread->poll_shrink = IOTHREAD_POLL_SHRINK_DEFAULT;
iothread->poll_weight = IOTHREAD_POLL_WEIGHT_DEFAULT;
iothread->thread_id = -1;
qemu_sem_init(&iothread->init_done_sem, 0);
/* By default, we don't run gcontext */
@@ -164,6 +157,7 @@ static void iothread_set_aio_context_params(EventLoopBase *base, Error **errp)
iothread->poll_max_ns,
iothread->poll_grow,
iothread->poll_shrink,
iothread->poll_weight,
errp);
if (*errp) {
return;
@@ -233,6 +227,9 @@ static IOThreadParamInfo poll_grow_info = {
static IOThreadParamInfo poll_shrink_info = {
"poll-shrink", offsetof(IOThread, poll_shrink),
};
static IOThreadParamInfo poll_weight_info = {
"poll-weight", offsetof(IOThread, poll_weight),
};
static void iothread_get_param(Object *obj, Visitor *v,
const char *name, IOThreadParamInfo *info, Error **errp)
@@ -254,13 +251,31 @@ static bool iothread_set_param(Object *obj, Visitor *v,
return false;
}
if (value < 0) {
if (info->offset == offsetof(IOThread, poll_weight)) {
if (value < 0 || value > 63) {
error_setg(errp, "%s value must be in range [0, 63]",
info->name);
return false;
}
} else if (value < 0) {
error_setg(errp, "%s value must be in range [0, %" PRId64 "]",
info->name, INT64_MAX);
return false;
}
if (value == 0) {
if (info->offset == offsetof(IOThread, poll_grow)) {
*field = IOTHREAD_POLL_GROW_DEFAULT;
} else if (info->offset == offsetof(IOThread, poll_shrink)) {
*field = IOTHREAD_POLL_SHRINK_DEFAULT;
} else if (info->offset == offsetof(IOThread, poll_weight)) {
*field = IOTHREAD_POLL_WEIGHT_DEFAULT;
} else {
*field = value;
}
} else {
*field = value;
}
return true;
}
@@ -288,6 +303,7 @@ static void iothread_set_poll_param(Object *obj, Visitor *v,
iothread->poll_max_ns,
iothread->poll_grow,
iothread->poll_shrink,
iothread->poll_weight,
errp);
}
}
@@ -311,6 +327,10 @@ static void iothread_class_init(ObjectClass *klass, const void *class_data)
iothread_get_poll_param,
iothread_set_poll_param,
NULL, &poll_shrink_info);
object_class_property_add(klass, "poll-weight", "int",
iothread_get_poll_param,
iothread_set_poll_param,
NULL, &poll_weight_info);
}
static const TypeInfo iothread_info = {
@@ -356,6 +376,7 @@ 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->poll_weight = iothread->poll_weight;
info->aio_max_batch = iothread->parent_obj.aio_max_batch;
QAPI_LIST_APPEND(*tail, info);

1
monitor/hmp-cmds.c
View File

@@ -206,6 +206,7 @@ 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, " poll-weight=%" PRId64 "\n", value->poll_weight);
monitor_printf(mon, " aio-max-batch=%" PRId64 "\n",
value->aio_max_batch);
}

6
qapi/misc.json
View File

@@ -85,6 +85,11 @@
# @poll-shrink: how many ns will be removed from polling time, 0 means
# that it's not configured (since 2.9)
#
# @poll-weight: the weight factor for adaptive polling.
# Determines how much the current event interval contributes to
# the next polling time calculation. Valid values are 1 or
# greater (since 11.1)
#
# @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)
#
@@ -96,6 +101,7 @@
'poll-max-ns': 'int',
'poll-grow': 'int',
'poll-shrink': 'int',
'poll-weight': 'int',
'aio-max-batch': 'int' } }
##

10
qapi/qom.json
View File

@@ -606,6 +606,13 @@
# algorithm detects it is spending too long polling without
# encountering events. 0 selects a default behaviour (default: 0)
#
# @poll-weight: the weight factor for adaptive polling. Determines
# how much the most recent event interval affects the next
# polling duration calculation. If set to 0, the system default
# value of 3 is used. Typical values: 1 (high weight on recent
# interval), 2-4 (moderate weight on recent interval).
# (default: 0) (since 11.1)
#
# The @aio-max-batch option is available since 6.1.
#
# Since: 2.0
@@ -614,7 +621,8 @@
'base': 'EventLoopBaseProperties',
'data': { '*poll-max-ns': 'int',
'*poll-grow': 'int',
'*poll-shrink': 'int' } }
'*poll-shrink': 'int',
'*poll-weight': 'int' } }
##
# @MainLoopProperties:

8
qemu-options.hx
View File

@@ -6443,7 +6443,7 @@ 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,aio-max-batch=aio-max-batch``
``-object iothread,id=id,poll-max-ns=poll-max-ns,poll-grow=poll-grow,poll-shrink=poll-shrink,poll-weight=poll-weight,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.
@@ -6479,6 +6479,12 @@ SRST
the polling time when the algorithm detects it is spending too
long polling without encountering events.
The ``poll-weight`` parameter is the weight factor for adaptive
polling. It determines how much the most recent event interval
affects the next polling duration calculation. If set to 0, the
system default value of 3 is used. Typical values: 1 (high weight
on recent interval), 2-4 (moderate weight on recent interval).
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.

2
tests/unit/test-nested-aio-poll.c
View File

@@ -81,7 +81,7 @@ static void test(void)
qemu_set_current_aio_context(td.ctx);
/* Enable polling */
aio_context_set_poll_params(td.ctx, 1000000, 2, 2, &error_abort);
aio_context_set_poll_params(td.ctx, 1000000, 2, 2, 3, &error_abort);
/* Make the event notifier active (set) right away */
event_notifier_init(&td.poll_notifier, 1);

37
util/aio-posix.c
View File

@@ -29,7 +29,6 @@
/* Stop userspace polling on a handler if it isn't active for some time */
#define POLL_IDLE_INTERVAL_NS (7 * NANOSECONDS_PER_SECOND)
#define POLL_WEIGHT_SHIFT (3)
static void update_handler_poll_times(AioContext *ctx, int64_t block_ns,
int64_t dispatch_time);
@@ -582,28 +581,11 @@ static bool try_poll_mode(AioContext *ctx, AioHandlerList *ready_list,
static void adjust_polling_time(AioContext *ctx, int64_t block_ns)
{
if (block_ns < ctx->poll_ns) {
int64_t old = ctx->poll_ns;
int64_t shrink = ctx->poll_shrink;
if (shrink == 0) {
shrink = 2;
}
if (block_ns < (ctx->poll_ns / shrink)) {
ctx->poll_ns /= shrink;
}
trace_poll_shrink(ctx, old, ctx->poll_ns);
} else if (block_ns > ctx->poll_ns) {
if (block_ns > ctx->poll_ns) {
/* There is room to grow, poll longer */
int64_t old = ctx->poll_ns;
int64_t grow = ctx->poll_grow;
if (grow == 0) {
grow = 2;
}
if (block_ns > ctx->poll_ns * grow) {
ctx->poll_ns = block_ns;
} else {
@@ -615,6 +597,11 @@ static void adjust_polling_time(AioContext *ctx, int64_t block_ns)
}
trace_poll_grow(ctx, old, ctx->poll_ns);
} else if (block_ns < (ctx->poll_ns / ctx->poll_shrink)) {
int64_t old = ctx->poll_ns;
ctx->poll_ns /= ctx->poll_shrink;
trace_poll_shrink(ctx, old, ctx->poll_ns);
}
}
@@ -632,8 +619,8 @@ static void update_handler_poll_times(AioContext *ctx, int64_t block_ns,
* block_ns and previous poll.ns to smooth adjustments.
*/
node->poll.ns = node->poll.ns
? (node->poll.ns - (node->poll.ns >> POLL_WEIGHT_SHIFT))
+ (block_ns >> POLL_WEIGHT_SHIFT) : block_ns;
? (node->poll.ns - (node->poll.ns >> ctx->poll_weight))
+ (block_ns >> ctx->poll_weight) : block_ns;
if (node->poll.ns > ctx->poll_max_ns) {
node->poll.ns = 0;
@@ -819,7 +806,8 @@ void aio_context_destroy(AioContext *ctx)
}
void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
int64_t grow, int64_t shrink, Error **errp)
int64_t grow, int64_t shrink,
int64_t weight, Error **errp)
{
AioHandler *node;
@@ -833,8 +821,9 @@ void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
* is used once.
*/
ctx->poll_max_ns = max_ns;
ctx->poll_grow = grow;
ctx->poll_shrink = shrink;
ctx->poll_grow = (grow ? grow : IOTHREAD_POLL_GROW_DEFAULT);
ctx->poll_shrink = (shrink ? shrink : IOTHREAD_POLL_SHRINK_DEFAULT);
ctx->poll_weight = (weight ? weight : IOTHREAD_POLL_WEIGHT_DEFAULT);
ctx->poll_ns = 0;
aio_notify(ctx);

3
util/aio-win32.c
View File

@@ -429,7 +429,8 @@ void aio_context_destroy(AioContext *ctx)
}
void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
int64_t grow, int64_t shrink, Error **errp)
int64_t grow, int64_t shrink,
int64_t weight, Error **errp)
{
if (max_ns) {
error_setg(errp, "AioContext polling is not implemented on Windows");

1
util/async.c
View File

@@ -609,6 +609,7 @@ AioContext *aio_context_new(Error **errp)
ctx->poll_ns = 0;
ctx->poll_grow = 0;
ctx->poll_shrink = 0;
ctx->poll_weight = 0;
ctx->aio_max_batch = 0;