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[80.187.83.189]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4255d8a6daasm33593052f8f.7.2025.10.09.00.23.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 09 Oct 2025 00:23:52 -0700 (PDT) Message-ID: <8a65e8c1-3028-4f9c-bf1f-c8f1d3956192@redhat.com> Date: Thu, 9 Oct 2025 09:23:49 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Bug: Performance regression in 1013af4f585f: mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race To: Prakash Sangappa Cc: Jann Horn , "Uschakow, Stanislav" , "linux-mm@kvack.org" , "trix@redhat.com" , "ndesaulniers@google.com" , "nathan@kernel.org" , "akpm@linux-foundation.org" , "muchun.song@linux.dev" , "mike.kravetz@oracle.com" , Lorenzo Stoakes , Liam Howlett , "osalvador@suse.de" , "vbabka@suse.cz" , "stable@vger.kernel.org" References: <4d3878531c76479d9f8ca9789dc6485d@amazon.de> <2dcf12d0-e29c-4c9b-aeac-a0b803d2c2fd@redhat.com> <805DB7B5-23C2-437F-BB94-2188E310FD75@oracle.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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On large machines with 1.5TB of memory with 196 cores, we identified mmapping of 1.2TB of shared memory and forking itself dozens or hundreds of times we see a increase of execution times of a factor of 4. The reproducer is at the end of the email. >>> Yeah, every 1G virtual address range you unshare on unmap will do an >>> extra synchronous IPI broadcast to all CPU cores, so it's not very >>> surprising that doing this would be a bit slow on a machine with 196 >>> cores. >> >> What is the use case for this extreme usage of fork() in that context? Is it just something people noticed and it's suboptimal, or is this a real problem for some use cases? > > Our DB team is reporting performance issues due to this change. While running TPCC, Database > timeouts & shuts down(crashes). This is seen when there are a large number of > processes(thousands) involved. It is not so prominent when there are lesser number of > processes. > > Backing out this change addresses the problem. I suspect the timeouts are due to fork() taking longer, and there is no kernel crash etc, right? -- Cheers David / dhildenb