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Fri, 03 Nov 2023 08:05:09 -0400 Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-149-LB7Hzr_wOI-Llf_vhby0Og-1; Fri, 03 Nov 2023 08:05:03 -0400 Received: from smtp.corp.redhat.com (int-mx09.intmail.prod.int.rdu2.redhat.com [10.11.54.9]) (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 mimecast-mx02.redhat.com (Postfix) with ESMTPS id DDAB288D287; Fri, 3 Nov 2023 12:05:02 +0000 (UTC) Received: from secure.mitica (unknown [10.39.195.26]) by smtp.corp.redhat.com (Postfix) with ESMTP id 5A95C492BE0; Fri, 3 Nov 2023 12:05:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1699013105; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=OuumQ6CMhzFrUiWF8J0EXQY2x/d5itSYYSukSJVzVFw=; b=BuP1yjAyXDG3+9qPNQnZcOzyqDb58EFFd1FqqSOTsfVsu1piLpZuCJJvfCqhxlicTL7a/R PrjT3SQcp8hYQo2AyELkqyV3tiZVWev0LjiSi9GAsHhOATXOXQ9l2KeLVBJmmxvuXrDx55 bDpdMnkmPyDSiFhEcXZw9HKgNrEm1MM= X-MC-Unique: LB7Hzr_wOI-Llf_vhby0Og-1 From: Juan Quintela To: qemu-devel@nongnu.org Cc: Hyman Huang , Thomas Huth , Peter Xu , Paolo Bonzini , Fabiano Rosas , Laurent Vivier , Juan Quintela , Leonardo Bras Subject: [PULL 6/7] docs/migration: Add the dirty limit section Date: Fri, 3 Nov 2023 13:04:47 +0100 Message-ID: <20231103120448.58428-7-quintela@redhat.com> In-Reply-To: <20231103120448.58428-1-quintela@redhat.com> References: <20231103120448.58428-1-quintela@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.9 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.129.124; envelope-from=quintela@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -25 X-Spam_score: -2.6 X-Spam_bar: -- X-Spam_report: (-2.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.47, 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_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, 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+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1699013156398100010 Content-Type: text/plain; charset="utf-8" From: Hyman Huang The dirty limit feature has been introduced since the 8.1 QEMU release but has not reflected in the document, add a section for that. Signed-off-by: Hyman Huang Reviewed-by: Fabiano Rosas Reviewed-by: Juan Quintela Signed-off-by: Juan Quintela Message-ID: <0f2b2c63fec22ea23e4926cdeb567b7a0ebd8152.1698847223.git.yong.h= uang@smartx.com> --- docs/devel/migration.rst | 71 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 71 insertions(+) diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst index 240eb16d90..5adf4f12f7 100644 --- a/docs/devel/migration.rst +++ b/docs/devel/migration.rst @@ -594,6 +594,77 @@ path. Return path - opened by main thread, written by main thread AND post= copy thread (protected by rp_mutex) =20 +Dirty limit +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +The dirty limit, short for dirty page rate upper limit, is a new capability +introduced in the 8.1 QEMU release that uses a new algorithm based on the = KVM +dirty ring to throttle down the guest during live migration. + +The algorithm framework is as follows: + +:: + + ------------------------------------------------------------------------= ------ + main --------------> throttle thread ------------> PREPARE(1) <-------- + thread \ | | + \ | | + \ V | + -\ CALCULATE(2) | + \ | | + \ | | + \ V | + \ SET PENALTY(3) ----- + -\ | + \ | + \ V + -> virtual CPU thread -------> ACCEPT PENALTY(4) + ------------------------------------------------------------------------= ------ + +When the qmp command qmp_set_vcpu_dirty_limit is called for the first time, +the QEMU main thread starts the throttle thread. The throttle thread, once +launched, executes the loop, which consists of three steps: + + - PREPARE (1) + + The entire work of PREPARE (1) is preparation for the second stage, + CALCULATE(2), as the name implies. It involves preparing the dirty + page rate value and the corresponding upper limit of the VM: + The dirty page rate is calculated via the KVM dirty ring mechanism, + which tells QEMU how many dirty pages a virtual CPU has had since the + last KVM_EXIT_DIRTY_RING_FULL exception; The dirty page rate upper + limit is specified by caller, therefore fetch it directly. + + - CALCULATE (2) + + Calculate a suitable sleep period for each virtual CPU, which will be + used to determine the penalty for the target virtual CPU. The + computation must be done carefully in order to reduce the dirty page + rate progressively down to the upper limit without oscillation. To + achieve this, two strategies are provided: the first is to add or + subtract sleep time based on the ratio of the current dirty page rate + to the limit, which is used when the current dirty page rate is far + from the limit; the second is to add or subtract a fixed time when + the current dirty page rate is close to the limit. + + - SET PENALTY (3) + + Set the sleep time for each virtual CPU that should be penalized based + on the results of the calculation supplied by step CALCULATE (2). + +After completing the three above stages, the throttle thread loops back +to step PREPARE (1) until the dirty limit is reached. + +On the other hand, each virtual CPU thread reads the sleep duration and +sleeps in the path of the KVM_EXIT_DIRTY_RING_FULL exception handler, that +is ACCEPT PENALTY (4). Virtual CPUs tied with writing processes will +obviously exit to the path and get penalized, whereas virtual CPUs involved +with read processes will not. + +In summary, thanks to the KVM dirty ring technology, the dirty limit +algorithm will restrict virtual CPUs as needed to keep their dirty page +rate inside the limit. This leads to more steady reading performance during +live migration and can aid in improving large guest responsiveness. + Postcopy =3D=3D=3D=3D=3D=3D=3D=3D =20 --=20 2.41.0