[PATCH v2 0/7] thread lazy_init_name_hash

From: Jeff Hostetler <jeffhost@microsoft.com>

Version 2 of this patch series addresses the coding style issues, compile errors in non-threaded builds, and updated API documentation.

This patch series is a performance optimization for lazy_init_name_hash() in name-hash.c on very large repositories.

This change allows lazy_init_name_hash() to optionally use multiple threads when building the the_index.dir_hash and the_index.name_hash hashmaps. The original code path has been preserved and is used when the repo is small or the system does not have sufficient CPUs.

A helper command (t/helper/test-lazy-init-name-hash) was created to demonstrate performance differences and validate output. For example, use the '-p' option to compare both code paths on a large repo.

During our testing on the Windows source tree (3.1M files, 500K folders, 450MB index) with an 8 (logical) core machine, we reduced the runtime of lazy_init_name_hash() from 1.4 to 0.27 seconds.

This patch series replaces my earlier
     * jh/memihash-opt (2017-02-17) 5 commits
patch series.  This series is an improvement over the
original proposal because it completely isolates the changes
in name name-hash.c (rather than having parts in preload-index.c)
and eliminates the need to update/invalidate precomputed hash
values as cache_entries are changed.

Jeff Hostetler (7):
  name-hash: specify initial size for istate.dir_hash table
  hashmap: allow memihash computation to be continued
  hashmap: Add disallow_rehash setting
  hashmap: document memihash_cont, hashmap_disallow_rehash api
  name-hash: perf improvement for lazy_init_name_hash
  name-hash: add test-lazy-init-name-hash
  name-hash: add perf test for lazy_init_name_hash

 Documentation/technical/api-hashmap.txt |  22 ++
 Makefile                                |   1 +
 cache.h                                 |   1 +
 hashmap.c                               |  29 +-
 hashmap.h                               |  25 ++
 name-hash.c                             | 495 +++++++++++++++++++++++++++++++-
 t/helper/test-lazy-init-name-hash.c     | 264 +++++++++++++++++
 t/perf/p0004-lazy-init-name-hash.sh     |  19 ++
 8 files changed, 847 insertions(+), 9 deletions(-)
 create mode 100644 t/helper/test-lazy-init-name-hash.c
 create mode 100644 t/perf/p0004-lazy-init-name-hash.sh