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 77841C43334 for ; Mon, 4 Jul 2022 11:36:46 +0000 (UTC) Received: from localhost ([::1]:55176 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1o8KNh-0002Kk-HI for qemu-devel@archiver.kernel.org; Mon, 04 Jul 2022 07:36:45 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:39838) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1o8KL8-00082H-5T for qemu-devel@nongnu.org; Mon, 04 Jul 2022 07:34:09 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:40435) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1o8KL5-0000AJ-5y for qemu-devel@nongnu.org; Mon, 04 Jul 2022 07:34:04 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1656934442; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=s4Lx0kc/wsfVPk7290MImGzSt3WCdA7nmWMtK+v9Zjo=; b=QIOQubewxeZwnfsio7Cfrmu/ZZj9/92hAJ8c514lcOAzixfJvJopaA/vsihiEhH1BGJDbx VWKUuOXSm3MuOaic/e5qf6L3xdUiEaSCQgd9Dp8WXMUAazbjezh2ddVUnlXAaRYA+W+DHQ Qu3ldkcs//RR1tgdz2vTQbAPviEl6Hk= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-544-1GuwmOZ_O--uuQE3HqaRbQ-1; Mon, 04 Jul 2022 07:34:00 -0400 X-MC-Unique: 1GuwmOZ_O--uuQE3HqaRbQ-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 786451019C8A for ; Mon, 4 Jul 2022 11:34:00 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.195.112]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 45C1040474E1; Mon, 4 Jul 2022 11:34:00 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 14DA721E690D; Mon, 4 Jul 2022 13:33:59 +0200 (CEST) From: Markus Armbruster To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Cc: Leonardo Bras , Juan Quintela , "Dr. David Alan Gilbert" , Eric Blake , Peter Xu , qemu-devel@nongnu.org Subject: Re: [PATCH v2 2/3] Add zero-copy-copied migration stat References: <20220701155935.482503-1-leobras@redhat.com> <20220701155935.482503-3-leobras@redhat.com> <877d4tz9gx.fsf@pond.sub.org> Date: Mon, 04 Jul 2022 13:33:59 +0200 In-Reply-To: ("Daniel P. =?utf-8?Q?Berrang?= =?utf-8?Q?=C3=A9=22's?= message of "Mon, 4 Jul 2022 10:06:35 +0100") Message-ID: <877d4txgbc.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 2.84 on 10.11.54.2 Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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" Daniel P. Berrang=C3=A9 writes: > On Mon, Jul 04, 2022 at 08:18:54AM +0200, Markus Armbruster wrote: >> Leonardo Bras writes: >>=20 >> > Signed-off-by: Leonardo Bras >> > --- >> > qapi/migration.json | 5 ++++- >> > migration/migration.c | 1 + >> > monitor/hmp-cmds.c | 4 ++++ >> > 3 files changed, 9 insertions(+), 1 deletion(-) >> > >> > diff --git a/qapi/migration.json b/qapi/migration.json >> > index 7102e474a6..925f009868 100644 >> > --- a/qapi/migration.json >> > +++ b/qapi/migration.json >> > @@ -55,6 +55,9 @@ >> > # @postcopy-bytes: The number of bytes sent during the post-copy phase >> > # (since 7.0). >> > # >> > +# @zero-copy-copied: The number of zero-copy flushes that reported da= ta sent >> > +# using zero-copy that ended up being copied. (sin= ce 7.2) >>=20 >> The description feels awkward. What's a "zero-copy flush", and why >> should the user care? I figure what users care about is the number of >> all-zero pages we had to "copy", i.e. send the bulky way. Is this what >> @zero-copy-copied reports? > > MigrationCapability field @zero-copy-send instructs QEMU to try to > avoid copying data between userspace and kernel space when transmitting > RAM region. > > Even if the kernel supports zero copy, it is not guaranteed to happen, > it is merely a request to try. > > QEMU periodically (once per migration iteration) flushes outstanding > zero-copy requests and gets an indication back of whether any copies > took place or not. > > So this counter is a reflection of how many iterations resulted in > zero-copy not being fully honoured. Aha. Thanks! > IOW, ideally this counter will always be zero. If it is non-zero, > then the magnitude gives a very very very rough guide to what's > going on. If it is '1' then it was just a transient limitation. > If it matches the number of migration iterations, then it is a > more systemic limitation. I think we should rephrase the documentation in terms of migration iterations instead of "flushes", because the latter is a non-obvious term without a definition. > Incidentally, do we report the migration iteration count ? I > thought we did, but i'm not finding it now that I look. I was about to ask exactly that. I'm not sure the value of @zero-copy-copied can be usefully interpreted without.