From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id AA725FA3743 for ; Sat, 29 Oct 2022 08:11:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229788AbiJ2ILB (ORCPT ); Sat, 29 Oct 2022 04:11:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33320 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229553AbiJ2ILA (ORCPT ); Sat, 29 Oct 2022 04:11:00 -0400 Received: from szxga03-in.huawei.com (szxga03-in.huawei.com [45.249.212.189]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 07CA37B297; Sat, 29 Oct 2022 01:10:59 -0700 (PDT) Received: from dggpemm500020.china.huawei.com (unknown [172.30.72.57]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4MzsVH3rgzzFqSR; Sat, 29 Oct 2022 16:08:07 +0800 (CST) Received: from dggpemm500006.china.huawei.com (7.185.36.236) by dggpemm500020.china.huawei.com (7.185.36.49) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Sat, 29 Oct 2022 16:10:57 +0800 Received: from [10.174.178.55] (10.174.178.55) by dggpemm500006.china.huawei.com (7.185.36.236) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Sat, 29 Oct 2022 16:10:56 +0800 Subject: Re: [PATCH v7 00/11] kallsyms: Optimizes the performance of lookup symbols From: "Leizhen (ThunderTown)" To: Luis Chamberlain CC: Josh Poimboeuf , Jiri Kosina , Miroslav Benes , Petr Mladek , Joe Lawrence , , , Masahiro Yamada , Alexei Starovoitov , Jiri Olsa , Kees Cook , Andrew Morton , , Steven Rostedt , "Ingo Molnar" References: <20221017064950.2038-1-thunder.leizhen@huawei.com> <77f1c8f0-5e67-0e57-9285-15ba613044fb@huawei.com> <4f06547b-456f-e1ec-c535-16577f502ff1@huawei.com> Message-ID: <712fae84-aadc-7d29-f311-a3352bab6346@huawei.com> Date: Sat, 29 Oct 2022 16:10:44 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.178.55] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To dggpemm500006.china.huawei.com (7.185.36.236) X-CFilter-Loop: Reflected Precedence: bulk List-ID: On 2022/10/27 14:27, Leizhen (ThunderTown) wrote: > > > On 2022/10/27 11:26, Leizhen (ThunderTown) wrote: >> >> >> On 2022/10/27 3:03, Luis Chamberlain wrote: >>> On Wed, Oct 26, 2022 at 02:44:36PM +0800, Leizhen (ThunderTown) wrote: >>>> On 2022/10/26 1:53, Luis Chamberlain wrote: >>>>> This answers how we don't use a hash table, the question was *should* we >>>>> use one? >>>> >>>> I'm not the original author, and I can only answer now based on my understanding. Maybe >>>> the original author didn't think of the hash method, or he has weighed it out. >>>> >>>> Hash is a good solution if only performance is required and memory overhead is not >>>> considered. Using hash will increase the memory size by up to "4 * kallsyms_num_syms + >>>> 4 * ARRAY_SIZE(hashtable)" bytes, kallsyms_num_syms is about 1-2 million. > > Sorry, 1-2 million ==> 0.1~0.2 million > >>>> >>>> Because I don't know what hash algorithm will be used, the cost of generating the >>>> hash value corresponding to the symbol name is unknown now. But I think it's gonna >>>> be small. But it definitely needs a simpler algorithm, the tool needs to implement >>>> the same hash algorithm. >>> >>> For instance, you can look at evaluating if alloc_large_system_hash() would help. >> The following three hash algorithms are compared. The kernel is compiled by defconfig on arm64. |---------------------------------------------------------------------------------------| | | hash &= HASH_TABLE_SIZE - 1 | | | number of conflicts >= 1000 | |---------------------------------------------------------------------------------------| | ARRAY_SIZE(hash_table) | crc16 | jhash_one_at_a_time | string hash_32 | |---------------------------------------------------------------------------------------| | | 345b: 3905 | 0d40: 1013 | 4a4c: 6548 | | | 35bb: 1016 | 35ce: 6549 | 883a: 1015 | | 0x10000 | 385b: 6548 | 4440: 19126 | d05f: 19129 | | | f0ba: 19127 | 7ebe: 3916 | ecda: 3903 | |---------------------------------------------------------------------------------------| | | 0ba: 19168 | 440: 19165 | 05f: 19170 | | | 45b: 3955 | 5ce: 6577 | 83a: 1066 | | 0x1000 | 5bb: 1069 | d40: 1052 | a4c: 6609 | | | 85b: 6582 | ebe: 3938 | cda: 3924 | |---------------------------------------------------------------------------------------| Based on the above test results, I conclude that: 1. For the worst-case scenario, the three algorithms are not much different. But the kernel only implements crc16 and string hash_32. The latter is not processed byte-by-byte, so it is coupled with byte order and sizeof(long). So crc16 is the best choice. 2. For the worst-case scenario, there are almost 19K strings are mapped to the same hash value，just over 1/10 of the total. And with my current compression-then-comparison approach, it's 25-30 times faster. So there's still a need for my current approach, and they can be combined. if (nr_conflicts(key) >= CONST_N) { newname = compress(name); for_each_name_in_slot(key): compare(new_name) } else { for_each_name_in_slot(key): compare(name) } Above CONST_N can be roughly calculated: time_of_compress(name) + N * time_of_compare(new_name) <= N * time_of_compare(name) 3. For the worst-case scenario, there is no obvious difference between ARRAY_SIZE(hash_table) 0x10000 and 0x1000. So ARRAY_SIZE(hash_table)=0x1000 is enough. Statistic information: |------------------------------------------------------| | nr_conflicts(key) | ARRAY_SIZE(hash_table) | |------------------------------------------------------| | <= ? | 0x1000 | 0x10000 | |------------------------------------------------------| | 0 | 0 | 7821 | | 20 | 19 | 57375 | | 40 | 2419 | 124 | | 60 | 1343 | 70 | | 80 | 149 | 73 | | 100 | 38 | 49 | | 200 | 108 | 16 | | 400 | 14 | 2 | | 600 | 2 | 2 | | 800 | 0 | 0 | | 1000 | 0 | 0 | | 100000 | 4 | 4 | |------------------------------------------------------| Also, I re-calculated: Using hash will increase the memory size by up to "6 * kallsyms_num_syms + 4 * ARRAY_SIZE(hashtable)" |---- What I said earlier was 4 The increased size is close to 1 MB if CONFIG_KALLSYMS_ALL=y. Hi, Luis: For the reasons of the above-mentioned second conclusion. And except for patches 4-6, even if only the hash method is used, other patches and option "--lto-clang" in patch 6/11 are also needed. If you don't mind, I'd like to use hash at the next stage. The current patch set is already huge. >> OK, I found the right hash function. In this way, the tool does not need to consider >> the byte order. > > https://en.wikipedia.org/wiki/Jenkins_hash_function > > Let's go with jenkins_one_at_a_time_hash(), which looks simpler and doesn't even > have to think about sizeof(long). It seems to be closest to our current needs. > > uint32_t jenkins_one_at_a_time_hash(const uint8_t* key, size_t length) { > size_t i = 0; > uint32_t hash = 0; > > while (i != length) { > hash += key[i++]; > hash += hash << 10; > hash ^= hash >> 6; > } > hash += hash << 3; > hash ^= hash >> 11; > hash += hash << 15; > > return hash; > } > >> >> include/linux/stringhash.h >> >> /* >> * Version 1: one byte at a time. Example of use: >> * >> * unsigned long hash = init_name_hash; >> * while (*p) >> * hash = partial_name_hash(tolower(*p++), hash); >> * hash = end_name_hash(hash); >> >> >>> >>> Luis >>> . >>> >> > -- Regards, Zhen Lei