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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