index-pack: support multithreaded delta resolvingnd/threaded-index-pack

This puts delta resolving on each base on a separate thread, one base
cache per thread. Per-thread data is grouped in struct thread_local.
When running with nr_threads == 1, no pthreads calls are made. The
system essentially runs in non-thread mode.

An experiment on a Xeon 24 core machine with linux-2.6.git shows that
performance does not increase proportional to the number of cores. So
by default, we use maximum 3 cores. Some numbers with --threads from 1
to 16:

1..4
real    1m16.310s  0m48.183s  0m37.866s  0m32.834s
user    1m13.773s  1m15.537s  1m15.781s  1m16.233s
sys     0m2.480s   0m3.936s   0m4.448s   0m4.852s

5..8
real    0m33.170s  0m30.369s  0m28.406s  0m26.968s
user    1m31.474s  1m30.322s  1m29.562s  1m28.694s
sys     0m6.096s   0m6.268s   0m6.684s   0m7.172s

9..12
real    0m26.288s  0m26.207s  0m26.239s  0m24.945s
user    1m29.530s  1m36.146s  1m43.134s  1m34.182s
sys     0m8.129s   0m8.437s   0m9.697s   0m10.201s

13..16
real    0m25.110s  0m25.043s  0m23.955s  0m25.746s
user    1m39.262s  1m43.598s  1m38.350s  1m59.775s
sys     0m10.997s  0m11.553s  0m11.949s  0m13.689s

Thanks to Ramsay Jones for troubleshooting on MinGW platform.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

0 files changed, 0 insertions, 0 deletions