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Thu, 10 Jul 2025 03:33:59 +0000 Message-ID: Date: Thu, 10 Jul 2025 11:33:46 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [RFC patch v3 00/20] Cache aware scheduling To: Madadi Vineeth Reddy , Tim Chen CC: Peter Zijlstra , Ingo Molnar , "K Prateek Nayak" , "Gautham R . Shenoy" , Juri Lelli , "Dietmar Eggemann" , Steven Rostedt , Ben Segall , Mel Gorman , "Valentin Schneider" , Tim Chen , "Vincent Guittot" , Libo Chen , "Abel Wu" , Hillf Danton , Len Brown , , References: <8f6c7c69-b6b3-4c82-8db3-96757f09245f@linux.ibm.com> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <8f6c7c69-b6b3-4c82-8db3-96757f09245f@linux.ibm.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SG2PR01CA0184.apcprd01.prod.exchangelabs.com (2603:1096:4:189::9) To DM4PR11MB6020.namprd11.prod.outlook.com (2603:10b6:8:61::19) 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: DM4PR11MB6020:EE_|PH8PR11MB8287:EE_ X-MS-Office365-Filtering-Correlation-Id: 436669f0-f833-4280-dccf-08ddbf629b60 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014|7416014; 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In the current >> implementation, threads within the same process are considered >> as entities that potentially share resources. > > [..snip..] > >> >> Comments and tests are much appreciated. > > When running ebizzy as below: > ebizzy -t 8 -S 10 > > I see ~24% degradation on the patched kernel, due to higher SMT2 and > SMT4 cycles compared to the baseline. ST cycles decreased. > > Since both P10 and P11 have LLC shared at the SMT4 level, even spawning > fewer threads easily crowds the LLC with the default llc_aggr_cap value > of 50. Increasing this value would likely make things worse, while > decreasing it to 25 effectively disables cache-aware scheduling > (as it limits selection to just one CPU). > > I understand that ebizzy itself doesn't benefit from cache sharing, so > it might not improve but here it actually *regresses*, and the impact > may be even larger on P10 /P11 because of its smaller LLC shared by 4 > CPUs, even with fewer threads. IPC drops. > > By default, the SCHED_CACHE feature is enabled. Given these results for > workloads that don't share cache and on systems with smaller LLCs, I think > the default value should be revisited. > Thanks for the test. I agree with you. The SMT number, the L3 cache size, the workload's working set size should all be considered to find a proper threshold to enable/disable task aggregation. thanks, Chenyu > Thanks, > Madadi Vineeth Reddy >