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From: Duy Nguyen <pclouds@gmail.com>
To: Ben Peart <peartben@gmail.com>
Cc: Jeff King <peff@peff.net>, Ben Peart <Ben.Peart@microsoft.com>,
	Git Mailing List <git@vger.kernel.org>,
	Junio C Hamano <gitster@pobox.com>
Subject: Re: [PATCH v1 0/3] [RFC] Speeding up checkout (and merge, rebase, etc)
Date: Tue, 24 Jul 2018 17:13:36 +0200	[thread overview]
Message-ID: <20180724151336.GA1957@duynguyen.home> (raw)
In-Reply-To: <6ff6fbdc-d9cf-019f-317c-7fdba31105c6@gmail.com>

On Mon, Jul 23, 2018 at 04:51:38PM -0400, Ben Peart wrote:
> >>> What's the current state of the index before this checkout?
> >>
> >> This was after running "git checkout" multiple times so there was really
> >> nothing for git to do.
> > 
> > Hmm.. this means cache-tree is fully valid, unless you have changes in
> > index. We're quite aggressive in repairing cache-tree since aecf567cbf
> > (cache-tree: create/update cache-tree on checkout - 2014-07-05). If we
> > have very good cache-tree records and still spend 33s on
> > traverse_trees, maybe there's something else.
> > 
> 
> I'm not at all familiar with the cache-tree and couldn't find any 
> documentation on it other than index-format.txt which says "it helps 
> speed up tree object generation for a new commit."

I guess you have the starting points you need after Jeff's and Junio's
explanation (and it would be great if cache-tree could actually be for
for this two-way merge). But to make it easier for new people in
future, maybe we should add this?

This is basically a ripoff of Junio's explanation with starting points
(write-tree and index-format.txt). I wanted to incorporate some pieces
from Jeff's too but I think Junio's already covered it well.

-- 8< --
Subject: [PATCH] cache-tree.h: more description of what it is and what's it used for

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 cache-tree.h | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/cache-tree.h b/cache-tree.h
index cfd5328cc9..d25a800a72 100644
--- a/cache-tree.h
+++ b/cache-tree.h
@@ -5,6 +5,35 @@
 #include "tree.h"
 #include "tree-walk.h"
 
+/*
+ * cache-tree is an index extension that records tree object names for
+ * subdirectories you see in the index. It is mainly used for
+ * generating trees from the index before you create a new commit (see
+ * builtin/write-tree.c as starting point) but it's also used in "git
+ * diff-index --cached $TREE" as an optimization. See index-format.txt
+ * for on-disk format.
+ *
+ * Every time you write the contents of the index as a tree object, we
+ * need to collect the object name for each top-level paths and write
+ * a new top-level tree object out, after doing the same recursively
+ * for any modified subdirectory. Whenever you add, remove or modify a
+ * path in the index, the cache-tree entry for enclosing directories
+ * are invalidated, so a cache-tree entry that is still valid means
+ * that all the paths in the index under that directory match the
+ * contents of the tree object that the cache-tree entry holds.
+ *
+ * And that property is used by "diff-index --cached $TREE" that is
+ * run internally.  When we find that the subdirectory "D"'s
+ * cache-tree entry is valid in the index, and the tree object
+ * recorded in the cache-tree for that subdirectory matches the
+ * subtree D in the tree object $TREE, then "diff-index --cached"
+ * ignores the entire subdirectory D (which saves relatively little in
+ * the index as it only needs to scan what is already in the memory
+ * forward, but on the $TREE traversal side, it does not have to even
+ * open a subtree, that can save a lot), and with a well-populated
+ * cache-tree, it can save a significant processing.
+ */
+
 struct cache_tree;
 struct cache_tree_sub {
 	struct cache_tree *cache_tree;
-- 
2.18.0.656.gda699b98b3

-- 8< --

  parent reply	other threads:[~2018-07-24 15:13 UTC|newest]

Thread overview: 121+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-07-18 20:45 [PATCH v1 0/3] [RFC] Speeding up checkout (and merge, rebase, etc) Ben Peart
2018-07-18 20:45 ` [PATCH v1 1/3] add unbounded Multi-Producer-Multi-Consumer queue Ben Peart
2018-07-18 20:57   ` Stefan Beller
2018-07-19 19:11   ` Junio C Hamano
2018-07-18 20:45 ` [PATCH v1 2/3] add performance tracing around traverse_trees() in unpack_trees() Ben Peart
2018-07-18 20:45 ` [PATCH v1 3/3] Add initial parallel version of unpack_trees() Ben Peart
2018-07-18 22:56   ` Junio C Hamano
2018-07-18 21:02 ` [PATCH v1 0/3] [RFC] Speeding up checkout (and merge, rebase, etc) Stefan Beller
2018-07-18 21:34 ` Jeff King
2018-07-23 15:48   ` Ben Peart
2018-07-23 17:03     ` Duy Nguyen
2018-07-23 20:51       ` Ben Peart
2018-07-24  4:20         ` Jeff King
2018-07-24 15:33           ` Duy Nguyen
2018-07-25 20:56             ` Ben Peart
2018-07-26  5:30               ` Duy Nguyen
2018-07-26 16:30                 ` Duy Nguyen
2018-07-26 19:40                   ` Junio C Hamano
2018-07-27 15:42                     ` Duy Nguyen
2018-07-27 16:22                       ` Ben Peart
2018-07-27 18:00                         ` Duy Nguyen
2018-07-27 17:14                       ` Junio C Hamano
2018-07-27 17:52                         ` Duy Nguyen
2018-07-29  6:24                           ` Duy Nguyen
2018-07-29 10:33                       ` [PATCH v2 0/4] Speed up unpack_trees() Nguyễn Thái Ngọc Duy
2018-07-29 10:33                         ` [PATCH v2 1/4] unpack-trees.c: add performance tracing Nguyễn Thái Ngọc Duy
2018-07-30 20:16                           ` Ben Peart
2018-07-29 10:33                         ` [PATCH v2 2/4] unpack-trees: optimize walking same trees with cache-tree Nguyễn Thái Ngọc Duy
2018-07-30 20:52                           ` Ben Peart
2018-07-29 10:33                         ` [PATCH v2 3/4] unpack-trees: reduce malloc in cache-tree walk Nguyễn Thái Ngọc Duy
2018-07-30 20:58                           ` Ben Peart
2018-07-29 10:33                         ` [PATCH v2 4/4] unpack-trees: cheaper index update when walking by cache-tree Nguyễn Thái Ngọc Duy
2018-08-08 18:46                           ` Elijah Newren
2018-08-10 16:39                             ` Duy Nguyen
2018-08-10 18:39                               ` Elijah Newren
2018-08-10 19:30                                 ` Duy Nguyen
2018-08-10 19:40                                   ` Elijah Newren
2018-08-10 19:48                                     ` Duy Nguyen
2018-07-30 18:10                         ` [PATCH v2 0/4] Speed up unpack_trees() Ben Peart
2018-07-31 15:31                           ` Duy Nguyen
2018-07-31 16:50                             ` Ben Peart
2018-07-31 17:31                               ` Ben Peart
2018-08-01 16:38                                 ` Duy Nguyen
2018-08-08 20:53                                   ` Ben Peart
2018-08-09  8:16                                     ` Ben Peart
2018-08-10 16:08                                       ` Duy Nguyen
2018-08-10 15:51                                     ` Duy Nguyen
2018-07-30 21:04                         ` Ben Peart
2018-08-04  5:37                         ` [PATCH v3 " Nguyễn Thái Ngọc Duy
2018-08-04  5:37                           ` [PATCH v3 1/4] unpack-trees: add performance tracing Nguyễn Thái Ngọc Duy
2018-08-04  5:37                           ` [PATCH v3 2/4] unpack-trees: optimize walking same trees with cache-tree Nguyễn Thái Ngọc Duy
2018-08-08 18:23                             ` Elijah Newren
2018-08-10 16:29                               ` Duy Nguyen
2018-08-10 18:48                                 ` Elijah Newren
2018-08-04  5:37                           ` [PATCH v3 3/4] unpack-trees: reduce malloc in cache-tree walk Nguyễn Thái Ngọc Duy
2018-08-08 18:30                             ` Elijah Newren
2018-08-04  5:37                           ` [PATCH v3 4/4] unpack-trees: cheaper index update when walking by cache-tree Nguyễn Thái Ngọc Duy
2018-08-06 15:48                           ` [PATCH v3 0/4] Speed up unpack_trees() Junio C Hamano
2018-08-06 15:59                             ` Duy Nguyen
2018-08-06 18:59                               ` Junio C Hamano
2018-08-08 17:00                                 ` Ben Peart
2018-08-08 17:46                               ` Junio C Hamano
2018-08-08 18:12                                 ` Junio C Hamano
2018-08-08 18:39                                   ` Junio C Hamano
2018-08-10 16:53                                     ` Duy Nguyen
2018-08-12  8:15                           ` [PATCH v4 0/5] " Nguyễn Thái Ngọc Duy
2018-08-12  8:15                             ` [PATCH v4 1/5] trace.h: support nested performance tracing Nguyễn Thái Ngọc Duy
2018-08-13 18:39                               ` Ben Peart
2018-08-12  8:15                             ` [PATCH v4 2/5] unpack-trees: add " Nguyễn Thái Ngọc Duy
2018-08-12 10:05                               ` Thomas Adam
2018-08-13 18:50                                 ` Junio C Hamano
2018-08-13 18:44                               ` Ben Peart
2018-08-13 19:25                               ` Jeff King
2018-08-13 19:36                                 ` Stefan Beller
2018-08-13 20:11                                   ` Ben Peart
2018-08-13 19:52                                 ` Duy Nguyen
2018-08-13 21:47                                   ` Jeff King
2018-08-13 22:41                                 ` Junio C Hamano
2018-08-14 18:19                                   ` Jeff Hostetler
2018-08-14 18:32                                     ` Duy Nguyen
2018-08-14 18:44                                       ` Stefan Beller
2018-08-14 18:51                                         ` Duy Nguyen
2018-08-14 19:54                                           ` Jeff King
2018-08-14 20:52                                           ` Junio C Hamano
2018-08-15 16:32                                             ` Duy Nguyen
2018-08-15 18:28                                               ` Junio C Hamano
2018-08-14 20:14                                         ` Jeff Hostetler
2018-08-12  8:15                             ` [PATCH v4 3/5] unpack-trees: optimize walking same trees with cache-tree Nguyễn Thái Ngọc Duy
2018-08-13 18:58                               ` Ben Peart
2018-08-15 16:38                                 ` Duy Nguyen
2018-08-12  8:15                             ` [PATCH v4 4/5] unpack-trees: reduce malloc in cache-tree walk Nguyễn Thái Ngọc Duy
2018-08-12  8:15                             ` [PATCH v4 5/5] unpack-trees: reuse (still valid) cache-tree from src_index Nguyễn Thái Ngọc Duy
2018-08-13 15:48                               ` Elijah Newren
2018-08-13 15:57                                 ` Duy Nguyen
2018-08-13 16:05                                 ` Ben Peart
2018-08-13 16:25                                   ` Duy Nguyen
2018-08-13 17:15                                     ` Ben Peart
2018-08-13 19:01                             ` [PATCH v4 0/5] Speed up unpack_trees() Junio C Hamano
2018-08-14 19:19                             ` Ben Peart
2018-08-18 14:41                             ` [PATCH v5 0/7] " Nguyễn Thái Ngọc Duy
2018-08-18 14:41                               ` [PATCH v5 1/7] trace.h: support nested performance tracing Nguyễn Thái Ngọc Duy
2018-08-18 14:41                               ` [PATCH v5 2/7] unpack-trees: add " Nguyễn Thái Ngọc Duy
2018-08-18 14:41                               ` [PATCH v5 3/7] unpack-trees: optimize walking same trees with cache-tree Nguyễn Thái Ngọc Duy
2018-08-20 12:43                                 ` Ben Peart
2018-08-18 14:41                               ` [PATCH v5 4/7] unpack-trees: reduce malloc in cache-tree walk Nguyễn Thái Ngọc Duy
2018-08-18 14:41                               ` [PATCH v5 5/7] unpack-trees: reuse (still valid) cache-tree from src_index Nguyễn Thái Ngọc Duy
2018-08-18 14:41                               ` [PATCH v5 6/7] unpack-trees: add missing cache invalidation Nguyễn Thái Ngọc Duy
2018-08-18 14:41                               ` [PATCH v5 7/7] cache-tree: verify valid cache-tree in the test suite Nguyễn Thái Ngọc Duy
2018-08-18 21:45                                 ` Elijah Newren
2018-08-18 22:01                               ` [PATCH v5 0/7] Speed up unpack_trees() Elijah Newren
2018-08-19  5:09                                 ` Duy Nguyen
2018-08-25 12:18                               ` [PATCH] Document update for nd/unpack-trees-with-cache-tree Nguyễn Thái Ngọc Duy
2018-08-25 12:31                                 ` Martin Ågren
2018-08-25 13:02                                 ` [PATCH v2] " Nguyễn Thái Ngọc Duy
2018-07-27 15:50                     ` [PATCH v1 0/3] [RFC] Speeding up checkout (and merge, rebase, etc) Ben Peart
2018-07-26 16:35               ` Duy Nguyen
2018-07-24  5:54         ` Junio C Hamano
2018-07-24 15:13         ` Duy Nguyen [this message]
2018-07-24 21:21           ` Jeff King
2018-07-25 16:09           ` Ben Peart
2018-07-24  4:27       ` Jeff King

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