From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on dcvr.yhbt.net X-Spam-Level: X-Spam-Status: No, score=-3.9 required=3.0 tests=AWL,BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, RCVD_IN_DNSWL_HI,SPF_HELO_PASS,SPF_PASS shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by dcvr.yhbt.net (Postfix) with ESMTP id 128AC1F9FD for ; Wed, 24 Feb 2021 19:14:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236136AbhBXTNr (ORCPT ); Wed, 24 Feb 2021 14:13:47 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59864 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236085AbhBXTMW (ORCPT ); Wed, 24 Feb 2021 14:12:22 -0500 Received: from mail-qv1-xf36.google.com (mail-qv1-xf36.google.com [IPv6:2607:f8b0:4864:20::f36]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8C851C0617AA for ; Wed, 24 Feb 2021 11:10:13 -0800 (PST) Received: by mail-qv1-xf36.google.com with SMTP id k5so1551328qvu.2 for ; Wed, 24 Feb 2021 11:10:13 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ttaylorr-com.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=OlvHIpN/0fZchJUQMEcQiUlDWg8Z7UeC1EeMZ372rD8=; b=krHC6OrtIAcmtJFNIPowaN1m7iNed3cvywjIJQA1MEnP60i/XReCeI4K7zm7guveCk l8du0uAUvRNZpuZFtARQ0chZRTkE/AwDVmw88ofgaQXBMivsEyyDzVADr35Xm/Wyg2Lk 7sYOyoruAcM+emEoq561waQBlWdUOrYwps2Ygmdky0c42BLaX+X1XlkFyoi97BOvu9aC vPv90wKmOtyVHktloFvTvtmqETndoGiYRZpz805NNJS+nPVfkFJnCbkjCIXQUXi5ePSg CK66iiRQJHfY+sud2XoKxQSTObdx16MzFy0g5hEiGNbPqcnC42LsiyDb7g2k+pvbj2K7 g+yA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=OlvHIpN/0fZchJUQMEcQiUlDWg8Z7UeC1EeMZ372rD8=; b=eNiI24k0TR0VYeQ8pzBAZ4QH1NNf0dBrJvI5aFc1UfAroshabbNKfDU+o4EfLSeJ1o cHRdIsxlPM+71RZzwVEMs5MYZtvEp8PvUQyvX18KE9iHtbGetLgbqB6Nay4kwi69LeZp cgIjDvzayB0dTDrG/pgkBQ0NksV3ylOzF6iCYqAIhfkn/I0hiidIFE0XKCKSUGhlTj+N jW8KEU1BfVeJoqWcwSItmk8zfFxIDEYwawge8IJmxnaxRJ3zipjELNLrifSEgH4ybwDX Y/O08znzQPtBkwp1L51POBpYsqTdrkO+ZlF5Ij7kiUGb3asm0XpXW2GZuX2spNvfcBmt mMyA== X-Gm-Message-State: AOAM533gx+341zEaR2d3boiJRc0rYxMXUiUXjJnT2lwQ4V8yl6NO8goz TGTFmVfxHbSDDLsAsurvzv4+Upwk2ay2RTD2 X-Google-Smtp-Source: ABdhPJwjFxU6JGXMhO3+GffB4sUXEJC8d4NvN/UC10Z23aJ2oLeHgMlEtB+DBEhs3C8r+0uUs0J+qw== X-Received: by 2002:a0c:e9c2:: with SMTP id q2mr23139282qvo.48.1614193812469; Wed, 24 Feb 2021 11:10:12 -0800 (PST) Received: from localhost ([2605:9480:22e:ff10:268b:c46e:d22e:db6b]) by smtp.gmail.com with ESMTPSA id m30sm849605qtd.30.2021.02.24.11.10.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 24 Feb 2021 11:10:12 -0800 (PST) Date: Wed, 24 Feb 2021 14:10:10 -0500 From: Taylor Blau To: git@vger.kernel.org Cc: peff@peff.net, dstolee@microsoft.com, avarab@gmail.com, gitster@pobox.com Subject: [PATCH v2 12/15] Documentation/technical: describe multi-pack reverse indexes Message-ID: <404d730498938da034d860d894ddbb7d6dffc27d.1614193703.git.me@ttaylorr.com> References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org As a prerequisite to implementing multi-pack bitmaps, motivate and describe the format and ordering of the multi-pack reverse index. The subsequent patch will implement reading this format, and the patch after that will implement writing it while producing a multi-pack index. Co-authored-by: Jeff King Signed-off-by: Jeff King Signed-off-by: Taylor Blau --- Documentation/technical/pack-format.txt | 80 +++++++++++++++++++++++++ 1 file changed, 80 insertions(+) diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt index 1faa949bf6..77eb591057 100644 --- a/Documentation/technical/pack-format.txt +++ b/Documentation/technical/pack-format.txt @@ -379,3 +379,83 @@ CHUNK DATA: TRAILER: Index checksum of the above contents. + +== multi-pack-index reverse indexes + +Similar to the pack-based reverse index, the multi-pack index can also +be used to generate a reverse index. + +Instead of mapping between offset, pack-, and index position, this +reverse index maps between an object's position within the MIDX, and +that object's position within a pseudo-pack that the MIDX describes. + +To clarify these three orderings, consider a multi-pack reachability +bitmap (which does not yet exist, but is what we are building towards +here). Each bit needs to correspond to an object in the MIDX, and so we +need an efficient mapping from bit position to MIDX position. + +One solution is to let bits occupy the same position in the oid-sorted +index stored by the MIDX. But because oids are effectively random, there +resulting reachability bitmaps would have no locality, and thus compress +poorly. (This is the reason that single-pack bitmaps use the pack +ordering, and not the .idx ordering, for the same purpose.) + +So we'd like to define an ordering for the whole MIDX based around +pack ordering, which has far better locality (and thus compresses more +efficiently). We can think of a pseudo-pack created by the concatenation +of all of the packs in the MIDX. E.g., if we had a MIDX with three packs +(a, b, c), with 10, 15, and 20 objects respectively, we can imagine an +ordering of the objects like: + + |a,0|a,1|...|a,9|b,0|b,1|...|b,14|c,0|c,1|...|c,19| + +where the ordering of the packs is defined by the MIDX's pack list, +and then the ordering of objects within each pack is the same as the +order in the actual packfile. + +Given the list of packs and their counts of objects, you can +naïvely reconstruct that pseudo-pack ordering (e.g., the object at +position 27 must be (c,1) because packs "a" and "b" consumed 25 of the +slots). But there's a catch. Objects may be duplicated between packs, in +which case the MIDX only stores one pointer to the object (and thus we'd +want only one slot in the bitmap). + +Callers could handle duplicates themselves by reading objects in order +of their bit-position, but that's linear in the number of objects, and +much too expensive for ordinary bitmap lookups. Building a reverse index +solves this, since it is the logical inverse of the index, and that +index has already removed duplicates. But, building a reverse index on +the fly can be expensive. Since we already have an on-disk format for +pack-based reverse indexes, let's reuse it for the MIDX's pseudo-pack, +too. + +Objects from the MIDX are ordered as follows to string together the +pseudo-pack. Let _pack(o)_ return the pack from which _o_ was selected +by the MIDX, and define an ordering of packs based on their numeric ID +(as stored by the MIDX). Let _offset(o)_ return the object offset of _o_ +within _pack(o)_. Then, compare _o~1~_ and _o~2~_ as follows: + + - If one of _pack(o~1~)_ and _pack(o~2~)_ is preferred and the other + is not, then the preferred one sorts first. ++ +(This is a detail that allows the MIDX bitmap to determine which +pack should be used by the pack-reuse mechanism, since it can ask +the MIDX for the pack containing the object at bit position 0). + + - If _pack(o~1~) ≠ pack(o~2~)_, then sort the two objects in + descending order based on the pack ID. + + - Otherwise, _pack(o~1~) = pack(o~2~)_, and the objects are + sorted in pack-order (i.e., _o~1~_ sorts ahead of _o~2~_ exactly + when _offset(o~1~) < offset(o~2~)_). + +In short, a MIDX's pseudo-pack is the de-duplicated concatenation of +objects in packs stored by the MIDX, laid out in pack order, and the +packs arranged in MIDX order (with the preferred pack coming first). + +Finally, note that the MIDX's reverse index is not stored as a chunk in +the multi-pack-index itself. This is done because the reverse index +includes the checksum of the pack or MIDX to which it belongs, which +makes it impossible to write in the MIDX. To avoid races when rewriting +the MIDX, a MIDX reverse index includes the MIDX's checksum in its +filename (e.g., `multi-pack-index-xyz.rev`). -- 2.30.0.667.g81c0cbc6fd