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 A8515C282DE for ; Tue, 11 Mar 2025 02:19:01 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1trpCH-0004oO-Ld; Mon, 10 Mar 2025 22:18:21 -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 1trpCG-0004ny-5p for qemu-devel@nongnu.org; Mon, 10 Mar 2025 22:18:20 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1trpCB-00061F-9g for qemu-devel@nongnu.org; Mon, 10 Mar 2025 22:18:19 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1741659493; 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=gHLIoOQm+QSB3Ye/VAzJHXxWu+NQy+2lmBGC2GDRK38=; b=SCHtq3MdGP/WpJORf55iqECoHy8UmBERhHYGH2t4+1cIgz8+h9gh+pjG9bibSyW9COe8if ZbhyzCiUBg3tdhlWCE7aOMzjUkuPiAYPrZr2xjFMJ6pAMNNNtbVR9fFXcYlRLw9txcXCtH MItVbtiCmrcE+yN7XamEqP6OE8nrZvU= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-516-DqTTIc7ONliFXAP4LpOfxA-1; Mon, 10 Mar 2025 22:18:11 -0400 X-MC-Unique: DqTTIc7ONliFXAP4LpOfxA-1 X-Mimecast-MFC-AGG-ID: DqTTIc7ONliFXAP4LpOfxA_1741659491 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (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-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 069EB1809CA5; Tue, 11 Mar 2025 02:18:10 +0000 (UTC) Received: from localhost (unknown [10.2.16.75]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id E75381800268; Tue, 11 Mar 2025 02:18:08 +0000 (UTC) Date: Tue, 11 Mar 2025 10:18:05 +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: <20250311021805.GC487512@fedora> References: <20250307221634.71951-1-kwolf@redhat.com> <20250307221634.71951-6-kwolf@redhat.com> <20250310105501.GC359802@fedora> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="InBOtQS0+G2YKmG1" Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 Received-SPF: pass client-ip=170.10.133.124; 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_H2=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 --InBOtQS0+G2YKmG1 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Mon, Mar 10, 2025 at 12:11:44PM +0100, Kevin Wolf wrote: > 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. > > >=20 > > > For example, think of a guest that tends to send a new I/O request so= on > > > 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 le= ast > > > 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 shrink= ing > > > 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 ju= st > > > fix the part that is easy enough to fix, we can revisit the rest late= r. > > >=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, = AioHandlerList *ready_list, > > > static bool try_poll_mode(AioContext *ctx, AioHandlerList *ready_lis= t, > > > 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) = - 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); > > > + } > > > + } > > > } > > > =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= **errp) > > > { > > > + AioHandler *node; > > > + > > > /* No thread synchronization here, it doesn't matter if an incor= rect value > > > * is used once. > > > */ > >=20 > > If you respin this series: > >=20 > > 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). > >=20 > > 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. >=20 > Yes, I can do that while applying the series. >=20 > Should I add your R-b after making the change? Yes, please. Stefan --InBOtQS0+G2YKmG1 Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmfPnV0ACgkQnKSrs4Gr c8ilcgf/aPI5LK5OGX/rbwCCIIrHNe9hvFFt4HtR9cgu7ZsoBUxY+QluZ0x2I+CN jGJacRmeUN8qYr9JXrDIYbtuNPEskI/9jd7Hd4vZX074ivGFUzvmq2eqw3tg73dO W3LRpdr17POBG9PN4xL/0jS2wUU7Mt7blLs5qC9mtwraqTCrJctpANES1USD+Of1 LeVS4kQO+dhabTb0JnzT8gI6fmVpoqiRFiD0J8Ql4/r079gb7uD+ogffIbktYFj5 MLdBX6YalrDVNyWROkp/iOHwNnNItYup3++M3Vek8XL1nz5ig0Wz90JnrUKXSWQe uspAPf5jkMoKj7ir/qDi2iKUlCNkYQ== =ZPOR -----END PGP SIGNATURE----- --InBOtQS0+G2YKmG1--