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 B38D7C4332F for ; Fri, 16 Dec 2022 15:31:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230465AbiLPPba (ORCPT ); Fri, 16 Dec 2022 10:31:30 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51038 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230086AbiLPPbZ (ORCPT ); Fri, 16 Dec 2022 10:31:25 -0500 Received: from mail-wm1-x32d.google.com (mail-wm1-x32d.google.com [IPv6:2a00:1450:4864:20::32d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2974711A0E for ; Fri, 16 Dec 2022 07:31:24 -0800 (PST) Received: by mail-wm1-x32d.google.com with SMTP id v124-20020a1cac82000000b003cf7a4ea2caso4350813wme.5 for ; Fri, 16 Dec 2022 07:31:24 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=cc:to:fcc:content-transfer-encoding:mime-version:subject:date:from :references:in-reply-to:message-id:from:to:cc:subject:date :message-id:reply-to; bh=Fbg90W1EzLJTTyG1F65DCePyJ3sPmIjFmGXqlXxzxd4=; b=IIQOj3o5XD7nvyZiGwKJ5eteoZYiw+rpt6Byr/zURinWgTLtjGY7dXeG5naifw8Ql5 59OEIurCy/6o8OKFXzR+Oy9q+mvDxRTvt4AVdky7JOCcjZRUSPJXnIuaFHMt0Ncr6AdR fetKhFfiJ6fT1v0XXCXd5WCeP+CsFuw+oWLuIpJt/9yMoj2VM38su+AyuZEZd0gTAUWA +iAroqZ+tJnZGqaMT7WwC51rd7lPptCGABB4SBEtOW5RvjBmhl1His+ufDyzQXqx0XFz C/xQOGUvEccJA69XGHfSysZ3SNKS3UU+2bWZyqGbQF4AS8QrVsvvl9CyXdL1ghZIiw/n kVTw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:fcc:content-transfer-encoding:mime-version:subject:date:from :references:in-reply-to:message-id:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=Fbg90W1EzLJTTyG1F65DCePyJ3sPmIjFmGXqlXxzxd4=; b=ZWQZ3JmprLpPX/un/mf7QUdxTyW2uQHAkCWxWXWlMiuYE6ldDM8cO7HsdyH7Fz9O93 bk7zTVqq7WahN7vIm2snwIBdWy1kyDwoDDPW+YCK/1c41dg9yb9xmP4PDvbZBtOCmHpS LXeSvXAUKZycjxR8B+joALwKbYHjDKg0i4+jeB2uEaL00I0f+nng1pUTqZu+5q0vD2vE To1R9NJLhQj7aeMnna5i0tp0oKxhr86Mx8IXEa3SQRW2LiWI6dvjPJrDz+q1imA9fDCQ RR3AgKgAWcBJ1bvXaox0ccpL2qO6y1G7r6FoI8pumhw/2i2p7P8p8z9RSwEC4gw2QQlk xE2A== X-Gm-Message-State: ANoB5pkMllLYbQ8HRX5C/v6n/OzihchwhRCsJbNUk0kujDKDmU+pmPPm /cVdHWQnmSt4L58O/ZzosEe0lBPQgVU= X-Google-Smtp-Source: AA0mqf7dWkfJdvwAzLOWDgdvJTEWZbeQxLuCzzIXuP86k19zjhgPq7lkVH9zDoZ9P8tpmx3Qs1WVuA== X-Received: by 2002:a05:600c:4f89:b0:3cf:d0be:1231 with SMTP id n9-20020a05600c4f8900b003cfd0be1231mr35884488wmq.13.1671204682465; Fri, 16 Dec 2022 07:31:22 -0800 (PST) Received: from [127.0.0.1] ([13.74.141.28]) by smtp.gmail.com with ESMTPSA id bi7-20020a05600c3d8700b003d1e051f671sm2843622wmb.9.2022.12.16.07.31.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 16 Dec 2022 07:31:22 -0800 (PST) Message-Id: In-Reply-To: References: From: "Derrick Stolee via GitGitGadget" Date: Fri, 16 Dec 2022 15:31:16 +0000 Subject: [PATCH v4 2/4] read-cache: add index.skipHash config option MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Fcc: Sent To: git@vger.kernel.org Cc: gitster@pobox.com, vdye@github.com, avarab@gmail.com, newren@gmail.com, Jacob Keller , Derrick Stolee , Derrick Stolee Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org From: Derrick Stolee The previous change allowed skipping the hashing portion of the hashwrite API, using it instead as a buffered write API. Disabling the hashwrite can be particularly helpful when the write operation is in a critical path. One such critical path is the writing of the index. This operation is so critical that the sparse index was created specifically to reduce the size of the index to make these writes (and reads) faster. This trade-off between file stability at rest and write-time performance is not easy to balance. The index is an interesting case for a couple reasons: 1. Writes block users. Writing the index takes place in many user- blocking foreground operations. The speed improvement directly impacts their use. Other file formats are typically written in the background (commit-graph, multi-pack-index) or are super-critical to correctness (pack-files). 2. Index files are short lived. It is rare that a user leaves an index for a long time with many staged changes. Outside of staged changes, the index can be completely destroyed and rewritten with minimal impact to the user. Following a similar approach to one used in the microsoft/git fork [1], add a new config option (index.skipHash) that allows disabling this hashing during the index write. The cost is that we can no longer validate the contents for corruption-at-rest using the trailing hash. [1] https://github.com/microsoft/git/commit/21fed2d91410f45d85279467f21d717a2db45201 We load this config from the repository config given by istate->repo, with a fallback to the_repository if it is not set. While older Git versions will not recognize the null hash as a special case, the file format itself is still being met in terms of its structure. Using this null hash will still allow Git operations to function across older versions. The one exception is 'git fsck' which checks the hash of the index file. This used to be a check on every index read, but was split out to just the index in a33fc72fe91 (read-cache: force_verify_index_checksum, 2017-04-14) and released first in Git 2.13.0. Document the versions that relaxed these restrictions, with the optimistic expectation that this change will be included in Git 2.40.0. Here, we disable this check if the trailing hash is all zeroes. We add a warning to the config option that this may cause undesirable behavior with older Git versions. As a quick comparison, I tested 'git update-index --force-write' with and without index.skipHash=true on a copy of the Linux kernel repository. Benchmark 1: with hash Time (mean ± σ): 46.3 ms ± 13.8 ms [User: 34.3 ms, System: 11.9 ms] Range (min … max): 34.3 ms … 79.1 ms 82 runs Benchmark 2: without hash Time (mean ± σ): 26.0 ms ± 7.9 ms [User: 11.8 ms, System: 14.2 ms] Range (min … max): 16.3 ms … 42.0 ms 69 runs Summary 'without hash' ran 1.78 ± 0.76 times faster than 'with hash' These performance benefits are substantial enough to allow users the ability to opt-in to this feature, even with the potential confusion with older 'git fsck' versions. It is critical that this test is placed before the test_index_version tests, since those tests obliterate the .git/config file and hence lose the setting from GIT_TEST_DEFAULT_HASH, if set. Signed-off-by: Derrick Stolee --- Documentation/config/index.txt | 11 +++++++++++ read-cache.c | 13 ++++++++++++- t/t1600-index.sh | 6 ++++++ 3 files changed, 29 insertions(+), 1 deletion(-) diff --git a/Documentation/config/index.txt b/Documentation/config/index.txt index 75f3a2d1054..23c7985eb40 100644 --- a/Documentation/config/index.txt +++ b/Documentation/config/index.txt @@ -30,3 +30,14 @@ index.version:: Specify the version with which new index files should be initialized. This does not affect existing repositories. If `feature.manyFiles` is enabled, then the default is 4. + +index.skipHash:: + When enabled, do not compute the trailing hash for the index file. + This accelerates Git commands that manipulate the index, such as + `git add`, `git commit`, or `git status`. Instead of storing the + checksum, write a trailing set of bytes with value zero, indicating + that the computation was skipped. ++ +If you enable `index.skipHash`, then Git clients older than 2.13.0 will +refuse to parse the index and Git clients older than 2.40.0 will report an +error during `git fsck`. diff --git a/read-cache.c b/read-cache.c index 46f5e497b14..fde0697873c 100644 --- a/read-cache.c +++ b/read-cache.c @@ -1817,6 +1817,8 @@ static int verify_hdr(const struct cache_header *hdr, unsigned long size) git_hash_ctx c; unsigned char hash[GIT_MAX_RAWSZ]; int hdr_version; + unsigned char *start, *end; + struct object_id oid; if (hdr->hdr_signature != htonl(CACHE_SIGNATURE)) return error(_("bad signature 0x%08x"), hdr->hdr_signature); @@ -1827,10 +1829,16 @@ static int verify_hdr(const struct cache_header *hdr, unsigned long size) if (!verify_index_checksum) return 0; + end = (unsigned char *)hdr + size; + start = end - the_hash_algo->rawsz; + oidread(&oid, start); + if (oideq(&oid, null_oid())) + return 0; + the_hash_algo->init_fn(&c); the_hash_algo->update_fn(&c, hdr, size - the_hash_algo->rawsz); the_hash_algo->final_fn(hash, &c); - if (!hasheq(hash, (unsigned char *)hdr + size - the_hash_algo->rawsz)) + if (!hasheq(hash, start)) return error(_("bad index file sha1 signature")); return 0; } @@ -2915,9 +2923,12 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile, int ieot_entries = 1; struct index_entry_offset_table *ieot = NULL; int nr, nr_threads; + struct repository *r = istate->repo ? istate->repo : the_repository; f = hashfd(tempfile->fd, tempfile->filename.buf); + repo_config_get_maybe_bool(r, "index.skiphash", &f->skip_hash); + for (i = removed = extended = 0; i < entries; i++) { if (cache[i]->ce_flags & CE_REMOVE) removed++; diff --git a/t/t1600-index.sh b/t/t1600-index.sh index 010989f90e6..45feb0fc5d8 100755 --- a/t/t1600-index.sh +++ b/t/t1600-index.sh @@ -65,6 +65,12 @@ test_expect_success 'out of bounds index.version issues warning' ' ) ' +test_expect_success 'index.skipHash config option' ' + rm -f .git/index && + git -c index.skipHash=true add a && + git fsck +' + test_index_version () { INDEX_VERSION_CONFIG=$1 && FEATURE_MANY_FILES=$2 && -- gitgitgadget