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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 14 Apr 2023 23:10:58.5472 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 4f9e2249-0a64-40f4-4f2a-08db3d3d81f4 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[63.35.35.123];Helo=[64aa7808-outbound-1.mta.getcheckrecipient.com] X-MS-Exchange-CrossTenant-AuthSource: DBAEUR03FT030.eop-EUR03.prod.protection.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB9PR08MB9732 X-Spam-Status: No, score=-4.9 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,FORGED_SPF_HELO,KAM_DMARC_NONE,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_PASS,SPF_NONE,TXREP,T_SCC_BODY_TEXT_LINE,UNPARSEABLE_RELAY autolearn=no autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: Hi Cupertino,=0A= =0A= > I had a test application demonstrating the problem.=0A= =0A= So I don't see the problem discussed of huge pages being=0A= split into small pages due to the guard page. If you ask for=0A= N huge pages of stack, you actually get either N-1 or N huge=0A= pages. That's irrespectively of the size of the guard page.=0A= =0A= If you get N-1 huge pages the first/last part of the stack consist=0A= of small pages. This depends on the alignment of the previous=0A= mmap by accident since the stack allocation code does not=0A= request huge pages (ie. you get transparent huge pages if they=0A= happen to fit in the mmap range).=0A= =0A= Even in your basic example you always get exactly 1 huge page=0A= for all threads when the mmap is aligned. Interestingly in the=0A= unaligned mmap case (ie. N-1 huge pages), the RSS size can=0A= actually end up twice as large as the RSS size of aligned huge=0A= pages (eg. if you use ~2MBytes of stack it will exactly fit in one=0A= huge page if aligned, so 2MB RSS, but if not aligned might use=0A= 1.9MB of small pages with the rest falling into the next huge=0A= page, making RSS ~4MB).=0A= =0A= So the underlying problem is that the stack allocation code doesn't=0A= really support huge pages. It's bad to get random huge pages in=0A= stack allocations based on alignment from unrelated mmap calls -=0A= it affects RSS size randomly as well as TLB misses etc. Overall it seems=0A= best to disable huge pages in stacks until this is fixed.=0A= =0A= Cheers,=0A= Wilco=