From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 8ACAAC282DE for ; Mon, 10 Mar 2025 10:55:37 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1tran0-00005w-K5; Mon, 10 Mar 2025 06:55:18 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1tramx-000056-SL for qemu-devel@nongnu.org; Mon, 10 Mar 2025 06:55:15 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1tramu-0005uu-Ku for qemu-devel@nongnu.org; Mon, 10 Mar 2025 06:55:15 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1741604110; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=fDcCQ7CuPUGFWVbwfBZZf401C3MuFweFWiev+Lm6a7o=; b=KTsr+vRIw/ZGJZwnUyj668qT897mYxjy9Og5QH3vBYA5yOhLdBTUR0TANZ/080WPoUymoL P6ya9vYQXpADSWZI45DpBtsXZdq7UxAvPtFiwK9UnKPBpborCpHj1Ma2nRS7clS3aul6ZL JWQaERBX6rf4xpEqRUiXNQk9mBiWj68= Received: from mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-321-1DGw1vQqMBOhVdG81xWKGg-1; Mon, 10 Mar 2025 06:55:07 -0400 X-MC-Unique: 1DGw1vQqMBOhVdG81xWKGg-1 X-Mimecast-MFC-AGG-ID: 1DGw1vQqMBOhVdG81xWKGg_1741604106 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 48185195609E; Mon, 10 Mar 2025 10:55:05 +0000 (UTC) Received: from localhost (unknown [10.2.16.19]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 8D2903000197; Mon, 10 Mar 2025 10:55:04 +0000 (UTC) Date: Mon, 10 Mar 2025 18:55:01 +0800 From: Stefan Hajnoczi To: Kevin Wolf Cc: qemu-block@nongnu.org, pbonzini@redhat.com, afaria@redhat.com, hreitz@redhat.com, qemu-devel@nongnu.org Subject: Re: [PATCH 5/5] aio-posix: Separate AioPolledEvent per AioHandler Message-ID: <20250310105501.GC359802@fedora> References: <20250307221634.71951-1-kwolf@redhat.com> <20250307221634.71951-6-kwolf@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="Y1yVTQc4elM+8F+z" Content-Disposition: inline In-Reply-To: <20250307221634.71951-6-kwolf@redhat.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.129.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org --Y1yVTQc4elM+8F+z Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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. >=20 > 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. >=20 > 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. >=20 > 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. >=20 > Signed-off-by: Kevin Wolf > --- > 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(-) >=20 > 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; > =20 > /* 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; > }; > =20 > /* 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, AioH= andlerList *ready_list, > static bool try_poll_mode(AioContext *ctx, AioHandlerList *ready_list, > int64_t *timeout) > { > + AioHandler *node; > int64_t max_ns; > =20 > if (QLIST_EMPTY_RCU(&ctx->poll_aio_handlers)) { > return false; > } > =20 > - max_ns =3D qemu_soonest_timeout(*timeout, ctx->poll.ns); > + max_ns =3D 0; > + QLIST_FOREACH(node, &ctx->poll_aio_handlers, node_poll) { > + max_ns =3D MAX(max_ns, node->poll.ns); > + } > + max_ns =3D 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) > =20 > /* Adjust polling time */ > if (ctx->poll_max_ns) { > + AioHandler *node; > int64_t block_ns =3D qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - st= art; > - 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); > + } > + } > } > =20 > progress |=3D 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 **e= rrp) > { > + 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 =66rom 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 =3D 0; > + qemu_lockcnt_inc(&ctx->list_lock); > + QLIST_FOREACH(node, &ctx->aio_handlers, node) { > + node->poll.ns =3D 0; > + } > + qemu_lockcnt_dec(&ctx->list_lock); > =20 > ctx->poll_max_ns =3D max_ns; > ctx->poll_grow =3D 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); > =20 > - ctx->poll.ns =3D 0; > - > ctx->poll_max_ns =3D 0; > ctx->poll_grow =3D 0; > ctx->poll_shrink =3D 0; > --=20 > 2.48.1 >=20 --Y1yVTQc4elM+8F+z Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmfOxQUACgkQnKSrs4Gr c8jxZggAte3v8BaJ2N5myWYqM4fNtJ+lrfwH+QsQmcXchD3bVBiP4oCt0ecy7rjo wlbbIis0LfCttMDG6BoqUvosonCVMaOWy7hnKFF6o2yQ16LGasI7ndmlIMPdc4Ve 3aQ2DGBCUU0TX7DbcPpfnhGZUeIxbcG0fprf5FL/EOVz2Z6+H4bZ+McXh3x3p+Tc y4GFCNLLyFvgFIjw1pqb3FVDWmBeQ1T4vAakELn/89LVVc7/yVV25jv+xA/cbGwD vgMGhqi0TbmE6Es03P7K3yiX9/VS+ZRXGe20hfuINDS1Z44UOEacUv+q8iEtEQRu Z1oUll2elg972vc0JXkCg4tyNKVzsA== =a0gj -----END PGP SIGNATURE----- --Y1yVTQc4elM+8F+z--