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=-3.8 required=3.0 tests=AWL,BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, 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 D76F41F8C2 for ; Wed, 10 Feb 2021 23:05:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233646AbhBJXEZ (ORCPT ); Wed, 10 Feb 2021 18:04:25 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39500 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233554AbhBJXDz (ORCPT ); Wed, 10 Feb 2021 18:03:55 -0500 Received: from mail-qt1-x82b.google.com (mail-qt1-x82b.google.com [IPv6:2607:f8b0:4864:20::82b]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 24EEAC061756 for ; Wed, 10 Feb 2021 15:03:15 -0800 (PST) Received: by mail-qt1-x82b.google.com with SMTP id w20so2907283qta.0 for ; Wed, 10 Feb 2021 15:03:15 -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=GO+/MaJhqNMc6Vq2gvQI6+qn9o0YJhq46OdH2wbWPzM=; b=IVGJh102SXWA3RpaD5ondhVo0/l6LgSPtTu9uLCB9byWukcTu4B+C9En178oSkXV11 td+e8ogo6EuTLGF7Q5GO/9sVwXV7BHv04tTm0J1/yGqZ+HGpVgQWUfvNkaPEDKWp+nq/ AGroSaMFh1UHY0hGRlPX8tDLk5TtMHUxNo34/AdMNJyqf1/eSQjaY2QseMzTNgoxooo5 ZGP7ts2lSNOjpjGDde2D4jQ0k2GcrzzcvZ8s8ae1KPzeh31non0CXAZkBYl3z4+nYJiH PO5qE0N/xaOsB4PkJiOPZVbJkKPH3K59s6pnPtIMdupuSZLsHekDBxMw1KJFOSrOHbEw iG+w== 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=GO+/MaJhqNMc6Vq2gvQI6+qn9o0YJhq46OdH2wbWPzM=; b=hvJ5AfcOhg/g/eTCcdN9g6DtyRruc1PVreX6xvDC8d+rykugykbTj/sM+kvkX1BX4H ifYg9XZPilRB0j002D4SwK5b0nU0PwTdKl3ZP2xpnDWSF05Kjtdt2NnDi0x869JJxsrb g/Rpx00anYHcMNy6g2oVUszJW6oSIlUl8XlWPvNcxM8WBGCruFvqCgyyOW+JfmVmpgyg oN01nKss88KMAplOatQdfHA3I5cZikOqHtUehO6QOUszufFG95gerMjWsG+lpba62Lga k8OhrlJP+aRYqOpgXjZJFgN1Ihbq7Ba6OoY3xvvpFaMIKIW16mdMUql+2nem8+D7rq3J abIw== X-Gm-Message-State: AOAM532ypvgDF7kAfRd5DqeMgb969zkW+3qtBDVhwFuSGD4wm90hJpjJ V/IgklOsJsGKbFNtfXMCOBE6d6DwL2g1ijhY X-Google-Smtp-Source: ABdhPJzsChOtVD1iyjmHdD3+9xK+JgKGKPhS5tmrwvPtrVOZtF1x9Kh5Qz89f80BYrUcTe8iFL4Jpg== X-Received: by 2002:a05:622a:42:: with SMTP id y2mr4909937qtw.186.1612998194025; Wed, 10 Feb 2021 15:03:14 -0800 (PST) Received: from localhost ([2605:9480:22e:ff10:2c3d:3179:bfad:c65]) by smtp.gmail.com with ESMTPSA id w38sm2308855qth.79.2021.02.10.15.03.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 10 Feb 2021 15:03:13 -0800 (PST) Date: Wed, 10 Feb 2021 18:03:11 -0500 From: Taylor Blau To: git@vger.kernel.org Cc: dstolee@microsoft.com, gitster@pobox.com, peff@peff.net Subject: [PATCH 6/9] Documentation/technical: describe multi-pack reverse indexes Message-ID: 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 | 83 +++++++++++++++++++++++++ 1 file changed, 83 insertions(+) diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt index 8833b71c8b..a14722f119 100644 --- a/Documentation/technical/pack-format.txt +++ b/Documentation/technical/pack-format.txt @@ -376,3 +376,86 @@ 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. +Crucially, the objects' positions within this pseudo-pack are the same +as their bit positions in a multi-pack reachability bitmap. + +As a motivating example, consider the multi-pack reachability bitmap +(which does not yet exist, but is what we are building towards here). We +need each bit to correspond to an object covered by the midx, and we +need to be able to convert bit positions back to index positions (from +which we can get the oid, etc). + +One solution is to let each bit position in the index correspond to +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. We can think of it as 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