On Fri, Mar 07, 2025 at 11:16:34PM +0100, Kevin Wolf wrote:
> Adaptive polling has a big problem: It doesn't consider that an event
> loop can wait for many different events that may have very different
> typical latencies.
>
> For example, think of a guest that tends to send a new I/O request soon
> after the previous I/O request completes, but the storage on the host is
> rather slow. In this case, getting the new request from guest quickly
> means that polling is enabled, but the next thing is performing the I/O
> request on the backend, which is slow and disables polling again for the
> next guest request. This means that in such a scenario, polling could
> help for every other event, but is only ever enabled when it can't
> succeed.
>
> In order to fix this, keep a separate AioPolledEvent for each
> AioHandler. We will then know that the backend file descriptor always
> has a high latency and isn't worth polling for, but we also know that
> the guest is always fast and we should poll for it. This solves at least
> half of the problem, we can now keep polling for those cases where it
> makes sense and get the improved performance from it.
>
> Since the event loop doesn't know which event will be next, we still do
> some unnecessary polling while we're waiting for the slow disk. I made
> some attempts to be more clever than just randomly growing and shrinking
> the polling time, and even to let callers be explicit about when they
> expect a new event, but so far this hasn't resulted in improved
> performance or even caused performance regressions. For now, let's just
> fix the part that is easy enough to fix, we can revisit the rest later.
>
> Signed-off-by: Kevin Wolf <kw...@redhat.com>
> ---
> include/block/aio.h | 1 -
> util/aio-posix.h | 1 +
> util/aio-posix.c | 24 +++++++++++++++++++++---
> util/async.c | 2 --
> 4 files changed, 22 insertions(+), 6 deletions(-)
>
> diff --git a/include/block/aio.h b/include/block/aio.h
> index 49f46e01cb..0ef7ce48e3 100644
> --- a/include/block/aio.h
> +++ b/include/block/aio.h
> @@ -233,7 +233,6 @@ struct AioContext {
> int poll_disable_cnt;
>
> /* Polling mode parameters */
> - AioPolledEvent poll;
> 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 */
> diff --git a/util/aio-posix.h b/util/aio-posix.h
> index 4264c518be..82a0201ea4 100644
> --- a/util/aio-posix.h
> +++ b/util/aio-posix.h
> @@ -38,6 +38,7 @@ struct AioHandler {
> #endif
> int64_t poll_idle_timeout; /* when to stop userspace polling */
> bool poll_ready; /* has polling detected an event? */
> + AioPolledEvent poll;
> };
>
> /* Add a handler to a ready list */
> diff --git a/util/aio-posix.c b/util/aio-posix.c
> index 259827c7ad..2251871c61 100644
> --- a/util/aio-posix.c
> +++ b/util/aio-posix.c
> @@ -579,13 +579,19 @@ static bool run_poll_handlers(AioContext *ctx,
> AioHandlerList *ready_list,
> static bool try_poll_mode(AioContext *ctx, AioHandlerList *ready_list,
> int64_t *timeout)
> {
> + AioHandler *node;
> int64_t max_ns;
>
> if (QLIST_EMPTY_RCU(&ctx->poll_aio_handlers)) {
> return false;
> }
>
> - max_ns = qemu_soonest_timeout(*timeout, ctx->poll.ns);
> + max_ns = 0;
> + QLIST_FOREACH(node, &ctx->poll_aio_handlers, node_poll) {
> + max_ns = MAX(max_ns, node->poll.ns);
> + }
> + max_ns = qemu_soonest_timeout(*timeout, max_ns);
> +
> if (max_ns && !ctx->fdmon_ops->need_wait(ctx)) {
> /*
> * Enable poll mode. It pairs with the poll_set_started() in
> @@ -721,8 +727,14 @@ bool aio_poll(AioContext *ctx, bool blocking)
>
> /* Adjust polling time */
> if (ctx->poll_max_ns) {
> + AioHandler *node;
> int64_t block_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - start;
> - adjust_polling_time(ctx, &ctx->poll, block_ns);
> +
> + QLIST_FOREACH(node, &ctx->poll_aio_handlers, node_poll) {
> + if (QLIST_IS_INSERTED(node, node_ready)) {
> + adjust_polling_time(ctx, &node->poll, block_ns);
> + }
> + }
> }
>
> progress |= aio_bh_poll(ctx);
> @@ -772,10 +784,16 @@ void aio_context_use_g_source(AioContext *ctx)
> void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
> int64_t grow, int64_t shrink, Error **errp)
> {
> + AioHandler *node;
> +
> /* No thread synchronization here, it doesn't matter if an incorrect
> value
> * is used once.
> */
If you respin this series:
This comment is confusing now that qemu_lockcnt_inc() is being used.
Lockcnt tells other threads in aio_set_fd_handler() not to remove nodes
from the aio_handlers list (because we're traversing the list).
The comment is about the poll state though, not about the aio_handlers
list. Moving it down to where poll_max_ns, etc are assigned would make
it clearer.
> - ctx->poll.ns = 0;
> + qemu_lockcnt_inc(&ctx->list_lock);
> + QLIST_FOREACH(node, &ctx->aio_handlers, node) {
> + node->poll.ns = 0;
> + }
> + qemu_lockcnt_dec(&ctx->list_lock);
>
> ctx->poll_max_ns = max_ns;
> ctx->poll_grow = grow;
> diff --git a/util/async.c b/util/async.c
> index 38667ea091..4124a948fd 100644
> --- a/util/async.c
> +++ b/util/async.c
> @@ -609,8 +609,6 @@ AioContext *aio_context_new(Error **errp)
> qemu_rec_mutex_init(&ctx->lock);
> timerlistgroup_init(&ctx->tlg, aio_timerlist_notify, ctx);
>
> - ctx->poll.ns = 0;
> -
> ctx->poll_max_ns = 0;
> ctx->poll_grow = 0;
> ctx->poll_shrink = 0;
> --
> 2.48.1
>
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