perf: add threading capability to perfbdiff Since we are releasing the GIL during diffing, it is interesting to see how a thread pool would perform on diffing. We add a new `--threads` argument to commands. Synchronizing the thread pool is a bit complex because we want to be able to reuse it from one run to another. On my computer (i7 with 4 cores + hyperthreading), I get the following data for about 12000 revisions: threads wall comb wall gain comb overhead none 31.596715 31.59 0.00% 0.00% 1 31.621228 31.62 -0.08% 0.09% 2 16.406202 32.8 48.08% 3.83% 3 11.598334 34.76 63.29% 10.03% 4 9.205421 36.77 70.87% 16.40% 5 8.517604 42.51 73.04% 34.57% 6 7.94645 47.58 74.85% 50.62% 7 7.434972 51.92 76.47% 64.36% 8 7.070638 55.34 77.62% 75.18% Compared to the feature disabled (threads=0), the overhead is negligible with the threading code (threads=1), and the gain is already 48% with two threads.

--- a/tests/test-contrib-perf.t
+++ b/tests/test-contrib-perf.t
@@ -175,7 +175,7 @@
   $ (testrepohg files -r 1.2 glob:mercurial/*.c glob:mercurial/*.py;
   >  testrepohg files -r tip glob:mercurial/*.c glob:mercurial/*.py) |
   > "$TESTDIR"/check-perf-code.py contrib/perf.py
-  contrib/perf.py:498:
+  contrib/perf.py:\d+: (re)
    >     from mercurial import (
    import newer module separately in try clause for early Mercurial
   [1]