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 9D09F1F9FD for ; Thu, 11 Mar 2021 17:06:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230000AbhCKRFz (ORCPT ); Thu, 11 Mar 2021 12:05:55 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36778 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230074AbhCKRF3 (ORCPT ); Thu, 11 Mar 2021 12:05:29 -0500 Received: from mail-io1-xd2a.google.com (mail-io1-xd2a.google.com [IPv6:2607:f8b0:4864:20::d2a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 16AD4C061574 for ; Thu, 11 Mar 2021 09:05:29 -0800 (PST) Received: by mail-io1-xd2a.google.com with SMTP id y20so4361470iot.4 for ; Thu, 11 Mar 2021 09:05:29 -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=G/wg4ROlEWl9gs4TbtB1/b/GUqHGdO2QzRK4YRW/sMs=; b=KEUpTeUZIbrL3CPpXFN7bKsYvLEtkNeS+2+aJvkr4RoKxfpLcDWOO7FlMEyUzs0nDF jCHsmmWAAE4D1YpjwacNla4GCWJqU9pwAF5iJ53r65dKmilQ01W70vpo1mU7/1g/xFYm BDWwCNUxAGXMHq9y8dXnLDM8Gjj52LqfeBQjs0yuzz38eZzewsPVgWbBx00fIrU2hKJF 4+9VD6ES6FuqmCiHsa62/Y8lpYPmHQ36kTq2WxuS+J67tkcRR8v62XeCtdtFYMSLDU1y gA0rTnHA9PFnbHf2i8NhWadPfm1AR6HCmmmsehwaq9dNrJ5YK0fexOSEZPMISLW7Ykc+ Vj2A== 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=G/wg4ROlEWl9gs4TbtB1/b/GUqHGdO2QzRK4YRW/sMs=; b=D9jt42UVdP6dAbm7g0k9j3MhKQ/4YnJMCit72bfE5Ta+ajj7icq2ddN52kJv+UcFwn ZSCtws/R4Vs7TzRVFCwowpE7sV6shnuAraPTRGDa//XGRF7xK+qEb6iXT1Oc51KrPLiy vM8XwzZvSc0otRlR8gN44BUg/qQzJnIjt7telFK6uC0uig3dYOpAUz+vyU2AhmuNkZzk iMhku87weW2C5PTNjbxWd0vlS5bjNNmJs3XltxJK2jRHx1L/maxMN/gwbjSLq5em6oUc gPa6roWtcnEW2iUnSWRn7XAR00XONqErtkvqu74dbyHzlVTqE9G8IfuwvgCCOC9ENs8i seFQ== X-Gm-Message-State: AOAM533UqxQVxnwMZMXCUae+tfwFV8mcIPs6Oul4aoWWqcq0tEWEy1B0 BG94SOU53owqrxUCx+zQPcExL1VqW5oHzI6M X-Google-Smtp-Source: ABdhPJwVggTGd+1eH6JVFmJdiGafkVyyhSW1I+U9AWXsMGnUSLHBMOaXQwCC9wOfhr661awScuTNrQ== X-Received: by 2002:a02:7086:: with SMTP id f128mr4578993jac.104.1615482328065; Thu, 11 Mar 2021 09:05:28 -0800 (PST) Received: from localhost ([2605:9480:22e:ff10:f947:1686:6ada:db5b]) by smtp.gmail.com with ESMTPSA id w9sm1724537iox.20.2021.03.11.09.05.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 11 Mar 2021 09:05:27 -0800 (PST) Date: Thu, 11 Mar 2021 12:05:25 -0500 From: Taylor Blau To: git@vger.kernel.org Cc: avarab@gmail.com, dstolee@microsoft.com, gitster@pobox.com, jonathantanmy@google.com, peff@peff.net Subject: [PATCH v3 12/16] Documentation/technical: describe multi-pack reverse indexes Message-ID: <4745bb8590f5cdc24445618dd63ba6bd541227b4.1615482270.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 | 83 +++++++++++++++++++++++++ 1 file changed, 83 insertions(+) diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt index 1faa949bf6..4bbbb188a4 100644 --- a/Documentation/technical/pack-format.txt +++ b/Documentation/technical/pack-format.txt @@ -379,3 +379,86 @@ 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 +(i.e., the ith entry of the multi-pack reverse index holds the MIDX +position of ith object in pseudo-pack order). + +To clarify the difference between these 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