Am 10.03.2025 um 11:55 hat Stefan Hajnoczi geschrieben: > 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.
Yes, I can do that while applying the series. Should I add your R-b after making the change? Kevin
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