From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C1C1240D575 for ; Wed, 29 Jul 2026 09:38:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785317925; cv=none; b=cSRHvMFZ82bBgf09odvTrE0XbvCK/nOvlxb+dqsasDHAL+9oZX4FU0gOxAzMf0y7GcOc5ZLUQxhvu6t8YMGo/cmP2JTzi7Zfs8Xx9HiIJVAlMfOt6KkBx/jl2aoYloDyk1ZekMagiYe2JiAJ/Z0ltAQqVJGui9SrwE0D7So3mNo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785317925; c=relaxed/simple; bh=HMwKdlQ2spGg5ZrgglubBMFICYLJYeEvWnrWq7jfT50=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=oFG8VRKEGf8c5vRYT2alS9XBrB2Ol2/mQ4KVayFEk2qVVAoap4E22+Z3omSP1Rr7KJ9mKhBWuJAe6ClZUyc7ikwPXAf9eh2tc38evIo14mUfttSuBqQpjB+ZBrjjs21k+i6xuSVZ0wYP5ohfDwK1kUU4607bhaQXI4Ww4kZBPZo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=W1aGByGa; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="W1aGByGa" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 50B841F000E9; Wed, 29 Jul 2026 09:38:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785317920; bh=eiNgAObNFAt6mBMIhHYbsPcfaTdqCcXt9eQhv43/YsY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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; 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charset=UTF-8 Content-Transfer-Encoding: 7bit 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