From mboxrd@z Thu Jan 1 00:00:00 1970 From: Linus Torvalds Subject: 'setup_work_tree()' considered harmful Date: Mon, 16 Jun 2008 17:45:25 -0700 (PDT) Message-ID: Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Cc: Johannes Schindelin To: Junio C Hamano , Git Mailing List X-From: git-owner@vger.kernel.org Tue Jun 17 02:46:37 2008 Return-path: Envelope-to: gcvg-git-2@gmane.org Received: from vger.kernel.org ([209.132.176.167]) by lo.gmane.org with esmtp (Exim 4.50) id 1K8PLH-0003xl-W5 for gcvg-git-2@gmane.org; Tue, 17 Jun 2008 02:46:36 +0200 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751977AbYFQApk (ORCPT ); Mon, 16 Jun 2008 20:45:40 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752202AbYFQApk (ORCPT ); Mon, 16 Jun 2008 20:45:40 -0400 Received: from smtp1.linux-foundation.org ([140.211.169.13]:50423 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750956AbYFQApj (ORCPT ); Mon, 16 Jun 2008 20:45:39 -0400 Received: from imap1.linux-foundation.org (imap1.linux-foundation.org [140.211.169.55]) by smtp1.linux-foundation.org (8.14.2/8.13.5/Debian-3ubuntu1.1) with ESMTP id m5H0jRiR019381 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Mon, 16 Jun 2008 17:45:28 -0700 Received: from localhost (localhost [127.0.0.1]) by imap1.linux-foundation.org (8.13.5.20060308/8.13.5/Debian-3ubuntu1.1) with ESMTP id m5H0jPqn022276; Mon, 16 Jun 2008 17:45:26 -0700 User-Agent: Alpine 1.10 (LFD 962 2008-03-14) X-Spam-Status: No, hits=-3.36 required=5 tests=AWL,BAYES_00 X-Spam-Checker-Version: SpamAssassin 3.2.4-osdl_revision__1.47__ X-MIMEDefang-Filter: lf$Revision: 1.188 $ X-Scanned-By: MIMEDefang 2.63 on 140.211.169.13 Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org Archived-At: [ Dscho cc'd because I think he is the primary culprits for this thing, I think. Commit e90fdc39b6903502192b2dd11e5503cea721a1ad in particular, methinks. ] So because I was looking at system call traces for object creation (due to those "rename to the final resting place" patches I did that got merged recently), I've noticed that some commands have *much* worse system call patterns than others. In particular, doing a "git add ." will use absolute pathnames for all git files, while a "git diff" will not. And this is quite noticeable - the absolute pathnames are not just longer, they have more path components in them. Making them a lot slower to look up and use. Of course, "a lot" depends on things a bit, but it really is noticeable. To test, I created a kernel tree (no .git), and did a "git init" followed by a "git add .". Here's the timings for current git head: [torvalds@woody kernel]$ time git add . real 0m8.377s user 0m6.556s sys 0m1.656s After that, I fixed "write_loose_object()" to not unnecessarily try to open a git object file, because every single caller has already done a "has_sha1_file(sha1)" or "has_loose_object(sha1)" check before calling that function, so trying to open it again is just pointless. As a result, git add sped up a tiny bit: [torvalds@woody kernel]$ time ~/git/git-add . real 0m8.341s user 0m6.588s sys 0m1.548s but I'll admit that it's not exactly noticeable. Half a percentage point is not a big deal. Then I created a hack that just made "setup_work_tree()" a no-op (because it does all that crazy stuff that forces GIT_DIR to be an absolute path etc). As a result I got: [torvalds@woody kernel]$ time ~/git/git-add . real 0m7.849s user 0m6.420s sys 0m1.296s ie now we're talking about a 5%+ performance difference. Of course, this is all for the hot-cache case, and it wouldn't be noticeable for a cold-cache case, but it really can be a real performance issue. And no, it's not that "setup_work_tree()" itself is expensive, but depending on how GIT_DIR is set up, you get very different system call patterns. Here's what current git does for one file (sound/usb/usbmixer_maps.c): lstat("sound/usb/usbmixer_maps.c", {st_mode=S_IFREG|0664, st_size=10230, ...}) = 0 open("sound/usb/usbmixer_maps.c", O_RDONLY) = 4 mmap(NULL, 10230, PROT_READ, MAP_PRIVATE, 4, 0) = 0x7fa247228000 close(4) = 0 access("/home/torvalds/kernel/.git/objects/d7/55be0ad8115da5c1296211678f81a7774277d5", F_OK) = -1 ENOENT (No such file or directory) open("/home/torvalds/kernel/.git/objects/d7/55be0ad8115da5c1296211678f81a7774277d5", O_RDONLY) = -1 ENOENT (No such file or directory) open("/home/torvalds/kernel/.git/objects/d7/tmp_obj_55be0ad8115da5c12962116_vDqXRl", O_RDWR|O_CREAT|O_EXCL, 0600) = 4 write(4, "x\1\315ZmS\333H\22\276\257\370W\364\361\t\210\1K\206$\300.\273\6\314\306\273@(l\'"..., 3402) = 3402 fchmod(4, 0444) = 0 close(4) = 0 link("/home/torvalds/kernel/.git/objects/d7/tmp_obj_55be0ad8115da5c12962116_vDqXRl", "/home/torvalds/kernel/.git/objects/d7/55be0ad8115da5c1296211678f81a7774277d5") = 0 unlink("/home/torvalds/kernel/.git/objects/d7/tmp_obj_55be0ad8115da5c12962116_vDqXRl") = 0 munmap(0x7fa247228000, 10230) = 0 and here is the fixed version: lstat("sound/usb/usbmixer_maps.c", {st_mode=S_IFREG|0664, st_size=10230, ...}) = 0 open("sound/usb/usbmixer_maps.c", O_RDONLY) = 4 mmap(NULL, 10230, PROT_READ, MAP_PRIVATE, 4, 0) = 0x7f54c231a000 close(4) = 0 access(".git/objects/d7/55be0ad8115da5c1296211678f81a7774277d5", F_OK) = -1 ENOENT (No such file or directory) open(".git/objects/d7/tmp_obj_55be0ad8115da5c12962116_fKy6px", O_RDWR|O_CREAT|O_EXCL, 0600) = 4 write(4, "x\1\315ZmS\333H\22\276\257\370W\364\361\t\210\1K\206$\300.\273\6\314\306\273@(l\'"..., 3402) = 3402 fchmod(4, 0444) = 0 close(4) = 0 link(".git/objects/d7/tmp_obj_55be0ad8115da5c12962116_fKy6px", ".git/objects/d7/55be0ad8115da5c1296211678f81a7774277d5") = 0 unlink(".git/objects/d7/tmp_obj_55be0ad8115da5c12962116_fKy6px") = 0 munmap(0x7f54c231a000, 10230) = 0 ie note how it does one unnecessary "open()" less, but more importantly, notice the difference between "/home/torvalds/kernel/.git/*" and ".git/*", and realize that that second difference was the much more noticeable one. And no, obviously the right fix is not to just comment out all of "setup_work_tree()" (it will break stuff that depends on GIT_WORKTREE), but I did that as a minimal example of showing what the bad effect of that function is. In general, I think we've gone in the wrong direction with a lot of the "make_absolute_path" stuff. See above. 5% performance loss is not good. Linus