Poor performance of git describe in big repos

["Alex Bennée" <kernel-hacker@bennee.com>]

Hi,

I'm a fairly heavy user of the magit Emacs extension for interacting
with my git repos. However I've noticed there are some cases where lag
is very high. By analysing strace output of emacs calling git I found
two commands that where particularly problematic when interrogating
the repo:

11:00 ajb@sloy/x86_64 [work.git] >time /usr/bin/git --no-pager
describe --long --tags
ajb-build-test-5224-10-gfa296e6

real    0m5.016s
user    0m4.364s
sys     0m0.444s

11:34 ajb@sloy/x86_64 [work.git] >time /usr/bin/git --no-pager
describe --contains HEAD
fatal: cannot describe 'fa296e61f549a1252a65a13b2f734d7afbc7e88e'

real    0m4.805s
user    0m4.388s
sys     0m0.400s

Running with first command with the --debug flag on gives:

11:34 ajb@sloy/x86_64 [work.git] >time /usr/bin/git --no-pager
describe --long --tags --debug
searching to describe HEAD
 lightweight       10 ajb-build-test-5224
 lightweight       41 ajb-build-test-5222
 annotated        146 vnms-2-1-36-32
 annotated        155 vnms-2-1-36-31
 annotated        174 vnms-2-1-36-30
 annotated        183 vnms-2-1-36-29
 lightweight      188 vnms-2-1-36-28
 annotated        193 vnms-2-1-36-27
 annotated        206 vnms-2-1-36-26
 annotated        215 vectastar-4-2-83-5
traversed 223 commits
more than 10 tags found; listed 10 most recent
gave up search at 2b69df72d47be8440e3ce4cee91b9b7ceaf8b77c
ajb-build-test-5224-10-gfa296e6

real    0m4.817s
user    0m4.320s
sys     0m0.464s

Which has only traversed 223 before coming to a decision. This seems
like a very low number of commits given the time it's spent doing
this.

One factor might be the size of my repo (.git is around 2.4G). Could
this just be due to computational cost of searching through large
packs to walk the commit chain? Is there any way to make this easier
for git to do?


-- 
Alex, homepage: http://www.bennee.com/~alex/