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x64; rv:91.0) Gecko/20100101 Thunderbird/91.9.0 Subject: Re: [PATCH 2/2] coroutine: Revert to constant batch size Content-Language: en-US To: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Cc: Kevin Wolf , "open list:Block layer core" , Stefan Hajnoczi , "qemu-devel@nongnu.org Developers" , qemu-stable References: <20220510151020.105528-1-kwolf@redhat.com> <20220510151020.105528-3-kwolf@redhat.com> <41c8f1d4-be27-4110-33c0-b53a701a9cb6@yahoo-corp.jp> From: =?UTF-8?B?5oiQ5bedIOW8mOaouQ==?= In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: TYXPR01CA0044.jpnprd01.prod.outlook.com (2603:1096:403:a::14) To TYWPR01MB10194.jpnprd01.prod.outlook.com (2603:1096:400:1e6::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 54f8787f-dffd-4c4a-58d5-08da347e7acc X-MS-TrafficTypeDiagnostic: OS3PR01MB9802:EE_ X-Microsoft-Antispam-PRVS: X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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envelope-from=hnarukaw@yahoo-corp.jp; helo=corp-ob10.yahoo-corp.jp 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, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-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+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" I'm not sure how much testing is expected for "Tested-by". If just checking my perspective is enough, yes. But I did not check that this patch fixes the problem of excessive resource consumption. On 2022/05/12 21:50, Philippe Mathieu-Daudé wrote: > Hi Hiroki, > > On Thu, May 12, 2022 at 8:57 AM 成川 弘樹 wrote: >> >> Thank you for your fix. >> >> I confirmed that after applying this patch, my intended performance >> improvement by 4c41c69e is still kept in our environment. > > Is that equivalent to a formal > Tested-by: Hiroki Narukawa > tag? > >> On 2022/05/11 0:10, Kevin Wolf wrote: >>> Commit 4c41c69e changed the way the coroutine pool is sized because for >>> virtio-blk devices with a large queue size and heavy I/O, it was just >>> too small and caused coroutines to be deleted and reallocated soon >>> afterwards. The change made the size dynamic based on the number of >>> queues and the queue size of virtio-blk devices. >>> >>> There are two important numbers here: Slightly simplified, when a >>> coroutine terminates, it is generally stored in the global release pool >>> up to a certain pool size, and if the pool is full, it is freed. >>> Conversely, when allocating a new coroutine, the coroutines in the >>> release pool are reused if the pool already has reached a certain >>> minimum size (the batch size), otherwise we allocate new coroutines. >>> >>> The problem after commit 4c41c69e is that it not only increases the >>> maximum pool size (which is the intended effect), but also the batch >>> size for reusing coroutines (which is a bug). It means that in cases >>> with many devices and/or a large queue size (which defaults to the >>> number of vcpus for virtio-blk-pci), many thousand coroutines could be >>> sitting in the release pool without being reused. >>> >>> This is not only a waste of memory and allocations, but it actually >>> makes the QEMU process likely to hit the vm.max_map_count limit on Linux >>> because each coroutine requires two mappings (its stack and the guard >>> page for the stack), causing it to abort() in qemu_alloc_stack() because >>> when the limit is hit, mprotect() starts to fail with ENOMEM. >>> >>> In order to fix the problem, change the batch size back to 64 to avoid >>> uselessly accumulating coroutines in the release pool, but keep the >>> dynamic maximum pool size so that coroutines aren't freed too early >>> in heavy I/O scenarios. >>> >>> Note that this fix doesn't strictly make it impossible to hit the limit, >>> but this would only happen if most of the coroutines are actually in use >>> at the same time, not just sitting in a pool. This is the same behaviour >>> as we already had before commit 4c41c69e. Fully preventing this would >>> require allowing qemu_coroutine_create() to return an error, but it >>> doesn't seem to be a scenario that people hit in practice. >>> >>> Cc: qemu-stable@nongnu.org >>> Resolves: https://jpn01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.redhat.com%2Fshow_bug.cgi%3Fid%3D2079938&data=05%7C01%7Chnarukaw%40yahoo-corp.jp%7Cff781f6380a7429d939208da3415f686%7Ca208d369cd4e4f87b11998eaf31df2c3%7C1%7C0%7C637879566175459621%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=1Sy3MvmkDTBSHZQxcEQUGZabBz%2FzTTYj4p0kFfTRTYM%3D&reserved=0 >>> Fixes: 4c41c69e05fe28c0f95f8abd2ebf407e95a4f04b >>> Signed-off-by: Kevin Wolf >>> ---