[PATCH 0/5] Suggested for PU: revision caching system to significantly speed up packing/walking

SUGGESTED FOR 'PU':

Traversing objects is currently very costly, as every commit and tree must be loaded and parsed. Much time and energy could be saved by caching metadata and topological info in an efficient, easily accessible manner. Furthermore, this could improve git's interfacing potential, by providing a condensed summary of a repository's commit tree.

This is a series to implement such a revision caching mechanism, aptly named 
rev-cache.  The series will provide:
 - a core API to manipulate and traverse caches
 - an integration into the internal revision walker
 - a porcelain front-end providing access to users and (shell) applications
 - a series of tests to verify/demonstrate correctness
 - documentation of the API, porcelain and core concepts

In cold starts rev-cache has sped up packing and walking by a factor of 4, and over twice that on warm starts. Some times on slax for the linux repository:

rev-list --all --objects >/dev/null
 default
   cold    1:13
   warm    0:43
 rev-cache'd
   cold    0:19
   warm    0:02

pack-objects --revs --all --stdout >/dev/null
 default
   cold    2:44
   warm    1:21
 rev-cache'd
   cold    0:44
   warm    0:10

The mechanism is minimally intrusive: most of the changes take place in seperate files, and only a handful of git's existing functions are modified.

Hope you find this useful.

 - Nick

 Documentation/rev-cache.txt           |   51 +
 Documentation/technical/rev-cache.txt |  336 ++++++
 Makefile                              |    2 +
 blob.c                                |    1 +
 blob.h                                |    1 +
 builtin-rev-cache.c                   |  284 +++++
 builtin.h                             |    1 +
 commit.c                              |    3 +
 commit.h                              |    2 +
 git.c                                 |    1 +
 list-objects.c                        |   49 +-
 rev-cache.c                           | 1832 +++++++++++++++++++++++++++++++++
 revision.c                            |   89 ++-
 revision.h                            |   46 +-
 t/t6015-rev-cache-list.sh             |  228 ++++
 t/t6015-sha1-dump-diff.py             |   36 +
 16 files changed, 2937 insertions(+), 25 deletions(-)
 create mode 100755 Documentation/rev-cache.txt
 create mode 100755 Documentation/technical/rev-cache.txt
 create mode 100755 builtin-rev-cache.c
 create mode 100755 rev-cache.c
 create mode 100755 t/t6015-rev-cache-list.sh
 create mode 100755 t/t6015-sha1-dump-diff.py