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 B00E4437847 for ; Thu, 30 Jul 2026 14:47:18 +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=1785422840; cv=none; b=d8dIJshBr3F5Y8+XB9mjDfKp6OXJHZpm2oIWuKakTtt2zjEVK7SIu3ZQM1oQcF5Kqsl3Hy4lGolhK8eQjj92rUerPtJKduX1AgcZysIj9PTf1uC1hpYKTA9FEI7aQQEfiEB5Pn4Ag2Be0XRLXvJoOalktBnPTQvfAeFv0kxvuOw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785422840; c=relaxed/simple; bh=Ivc/I8D+TN9ALdYBMmoIZSyV1mTWCY0bOI3pxMbSMBg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=aII4bU2Nt4Ub7ZkKyrQjY3x/ukbegkOZq4Kna4pCAHcIYbWPPxHmzwSi70KycJO42aExkqcy3Lcxwk0tYXtz0H10E3EXoYsoYEYCA0V2KWTjydoN/zUTc28jRnoXva+yvoYwv4ps2Yvw2ZxannFTzh/kw7opzX5Pz+d/CErmGRA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JGOrQ6Hq; 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="JGOrQ6Hq" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 853581F00A3D; Thu, 30 Jul 2026 14:47:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785422838; bh=6JNCYzO4mboOlKjltzKwN6FyPZkVxGE8uHEozODAlGM=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=JGOrQ6HqJK0w0NHZ5yY4kzE3rVaMpoL98iGN34RLoJwSAwUPYf7HzNprDrheGCRMj 3A7QZAeuLfn23Lj1c1bWd2ph6fFA166o9YdO4NVj2n1H+yMtfs/GV3bEzntQ70xp6M PrSYVIFl/o5aD9jIrRDnr9uozMxCBcNZlfL8jSBx2Ugp4PI1ybZBD75yA6BhhgPInB A72AXVcarQmxIkBModGhoAWmAFVK477sBoV7vLKxkKtUPykKvKNTT3ERZbDtJLqeoM IBsnn1qw+nOpLj9RXUMN0NQja6MK/Sr6HMD/kYM+/oQ/SyFp4mQdmU0xCcqKuiyxzm 8cBzRKPRHYV0A== Message-ID: <89e64c5d-f317-413e-9fa3-9b7e0a7f0f5f@kernel.org> Date: Thu, 30 Jul 2026 16:47:14 +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 Cc: Andrew Morton , Xu Xin , Chengming Zhou , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260716122039.679173-1-pedrodemargomes@gmail.com> <98c91508-4aa7-4390-b2f0-e39f16bdedef@kernel.org> 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 7/30/26 16:22, Pedro Demarchi Gomes wrote: > On Wed, Jul 29, 2026 at 11:38:38AM +0200, David Hildenbrand (Arm) wrote: >> 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! >> > > 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? >> > > As described in the KSM admin guide [1], the default values for pages_to_scan > and sleep_millisecs are intended for demonstration purposes rather than > production use. Well, I argue that echo 1 > /sys/kernel/mm/ksm/sleep_millisecs echo 100000 > /sys/kernel/mm/ksm/pages_to_scan is not for production use either? > > At LPC 2023, Stefan Roesch described the use of KSM in a production workload at > Meta [2] and presented optimizations such as Smart Scan and Advisor Mode, both > of which have since been merged to reduce KSM's scanning overhead. > > This RFC proposes a complementary optimization. KSM already computes a checksum > for every scanned page to identify frequently changing pages and avoid > unnecessary stable and unstable tree lookups. Reusing that checksum as an index > into those trees can further reduce lookup costs, lowering CPU usage without > changing KSM's behavior. Yes, but the change is not completely trivial, so we better make sure that the change is actually worth it in practice. -- Cheers, David