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b=W1aGByGa4skso+mkb+Im3FRmRn7Jkee/RrD9U3nU/R4b7aIjyGBRijQg+bv+/LnuL NDqgM0kRfXO9nnFrMbXoKUG+sNdHzI8c+TtJtzdTaQl2D3zqE/UK1LsuBo2YgPQ2NO V+27uYKEdCvZwbb1bH8WIirL9OLoU4wHQvkmh4hI6ejKy2LaTJr4qCdGknbSWxurCx Nqs7QT3IXuTUp+BHC/X1q2dH3b1rLGyIbM0+v3FMlsaGe1qxOu1aDwSuBoILS+I4Ye hmvgjShX5bBKsZETgDJa6m0QVqEFK/0sxDTEGoJhOFjZUZvzhFo554G++/tFy2YA5n zTmXR11uB3gcw== Message-ID: <98c91508-4aa7-4390-b2f0-e39f16bdedef@kernel.org> Date: Wed, 29 Jul 2026 11:38:38 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] mm/ksm: use checksum to speed up page comparison To: Pedro Demarchi Gomes , Andrew Morton , Xu Xin , Chengming Zhou Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260716122039.679173-1-pedrodemargomes@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20260716122039.679173-1-pedrodemargomes@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 51A3E140004 X-Rspamd-Server: rspam01 X-Stat-Signature: 9yqgdbn7tgenc47ui7km86eay18miikw X-HE-Tag: 1785317921-934870 X-HE-Meta: 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 LZLMsLi/ BTliBg2rHJQrinTdO1qTSXOy6J3z1qWiWH2BxyYG/Uvy05R+t8T8w21YNLB8XqK1VKA9L6H09HGzG1M6Lm1j4iQYmlwnqvHqdjrHHJg/8VN4aChKTIHhFNPNjrQ+pLPBxzMupMyJy9jpHsE9Q4sJyw/5EySXVZ044ZdbgONqHXVBsjjb1vGDbCjKdh4HnMF9aSKnr43Azh4f7L8f7EZ7tvAjoZPV2CjBOG1e5IsoOAi1Mlq4+W8GSBnUaGv5k8j1t+idi Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/16/26 14:20, Pedro Demarchi Gomes wrote: > Use page checksums as the primary ordering key when traversing the stable > and unstable trees and fall back to memcmp_pages() only when checksums > match. > Since struct ksm_stable_node does not have a checksum field, create one > in a union with migration list fields, so when we encounter a migration > page while scanning an address space we have to recalculate the page > checksum. This avoids increasing the size of struct ksm_stable_node, > which is maintained at 64 bytes, as show below. > > pedro@fedora:~/tmp/linux$ pahole -C ksm_stable_node ./vmlinux > struct ksm_stable_node { > union { > struct { > struct rb_node node __attribute__((__aligned__(8))); /* 0 24 */ > unsigned int checksum; /* 24 4 */ > } __attribute__((__aligned__(8))) __attribute__((__aligned__(8))); /* 0 32 */ > struct { > struct list_head * head; /* 0 8 */ > struct { > struct hlist_node hlist_dup; /* 8 16 */ > struct list_head list; /* 24 16 */ > }; /* 8 32 */ > }; /* 0 40 */ > } __attribute__((__aligned__(8))); /* 0 40 */ > struct hlist_head hlist; /* 40 8 */ > union { > long unsigned int kpfn; /* 48 8 */ > long unsigned int chain_prune_time; /* 48 8 */ > }; /* 48 8 */ > int rmap_hlist_len; /* 56 4 */ > int nid; /* 60 4 */ > > /* size: 64, cachelines: 1, members: 5 */ > /* forced alignments: 1 */ > } __attribute__((__aligned__(8))); > > To evaluate this change it was used two benchmarks, bench1.c and > bench2.c. The first one allocates 8G of pages with different content, > and the second one allocates 8G of pages where the first 4G are the same > as the last 4G. The two benchmarks and the system ksm configuration are > presented below. > > bench1.c: > > int main() { > size_t size = 8ULL * 1024*1024*1024; > unsigned long long int numpages = size/PAGESZ; > char *pages = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); > > // Generate #numpages pages with different contents > for (unsigned long long i = 0; i < numpages; i++) { > *((unsigned long long *) &pages[i*PAGESZ]) = i; > } > > if (madvise(pages, size, MADV_MERGEABLE) != 0) { > perror("madvise MADV_MERGEABLE failed"); > return 1; > } > printf("Wait...\n"); > getchar(); > return 0; > } > > bench2.c: > > int main() { > size_t size = 8ULL * 1024*1024*1024; > unsigned long long int numpages = size/PAGESZ; > char *pages = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); > > // Generate #numpages pages with different contents > for (unsigned long long i = 0; i < numpages/2; i++) { > *((unsigned long long *) &pages[i*PAGESZ]) = i; > *((unsigned long long *) &pages[(numpages-i-1)*PAGESZ]) = i; > } > > if (madvise(pages, size, MADV_MERGEABLE) != 0) { > perror("madvise MADV_MERGEABLE failed"); > return 1; > } > printf("Wait...\n"); > getchar(); > return 0; > } > Hi! > Configuration: > > echo never > /sys/kernel/mm/transparent_hugepage/enabled > echo 1 > /sys/kernel/mm/ksm/sleep_millisecs > echo 100000 > /sys/kernel/mm/ksm/pages_to_scan Given that the default is 100, and sleep_millisecs is 200 ... and it is known that frequent scanning is harmful for performance, what is the real world impact in common setups? IOW, do we even notice / care? -- Cheers, David