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2025 19:02:40 +0000 Message-ID: <3ad05298-351e-4d61-9972-ca45a0a50e33@meta.com> Date: Tue, 3 Jun 2025 15:00:43 -0400 User-Agent: Mozilla Thunderbird Content-Language: en-US From: Chris Mason To: Sebastian Andrzej Siewior , Peter Zijlstra , linux-kernel@vger.kernel.org Subject: futex performance regression from "futex: Allow automatic allocation of process wide futex hash" Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: BL0PR02CA0017.namprd02.prod.outlook.com (2603:10b6:207:3c::30) To LV3PR15MB6455.namprd15.prod.outlook.com (2603:10b6:408:1ad::10) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: LV3PR15MB6455:EE_|BL1PR15MB5314:EE_ X-MS-Office365-Filtering-Correlation-Id: 7318b713-2bee-4f0d-b948-08dda2d13633 X-FB-Source: Internal X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: 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a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=wKuvFiaSGQ0qltdbU6+NXLB8nM8=:19 a=Ol13hO9ccFRV9qXi2t6ftBPywas=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=6IFa9wvqVegA:10 a=NEAV23lmAAAA:8 a=VwQbUJbxAAAA:8 a=JfrnYn6hAAAA:8 a=1sWoMFGueSrtZMIvD1QA:9 a=QEXdDO2ut3YA:10 a=1CNFftbPRP8L7MoqJWF3:22 X-Proofpoint-ORIG-GUID: iARmZPi3Z2_lpsCwWTtN39b_0R757KBs X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1099,Hydra:6.0.736,FMLib:17.12.80.40 definitions=2025-06-03_02,2025-06-02_01,2025-03-28_01 Hi everyone, While testing Peter's latest scheduler patches against current Linus git, I found a pretty big performance regression with schbench: https://github.com/masoncl/schbench The command line I was using: schbench -L -m 4 -M auto -t 256 -n 0 -r 60 -s 0 Bisecting the problem I landed on commit: commit 7c4f75a21f636486d2969d9b6680403ea8483539 (HEAD -> update) Author: Sebastian Andrzej Siewior Date: Wed Apr 16 18:29:13 2025 +0200 futex: Allow automatic allocation of process wide futex hash Allocate a private futex hash with 16 slots if a task forks its first thread. Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Peter Zijlstra (Intel) Link: https://lore.kernel.org/r/20250416162921.513656-14bigeasy@linutronix.de schbench uses one futex per thread, and the command line ends up allocating 1024 threads, so the default bucket size used by this commit is just too small. Using 2048 buckets makes the problem go away. On my big turin system, this commit slows down RPS by 36%. But even a VM on a skylake machine sees a 29% difference. schbench is a microbenchmark, so grain of salt on all of this, but I think our defaults are probably too low. -chris