Rebase performance

Hi,

Using GIT_TRACE_PERFORMANCE it looks like a lot of time in a regular rebase is spent in run_apply() in builtin/am.c. This function first sets up a 'struct child_process cp' to launch "git apply" on a patch and then uses run_command(&cp) to actually launch the "git apply". Then this function calls discard_cache() and read_cache_from() to get the index created by the "git apply".

On a Linux server with many CPUs and many cores on each CPU, it is strange because the same rebase of 13 commits on a big repo is significantly slower than on a laptop (typically around 9 seconds versus 6 seconds). Both the server and the laptop have that has SSD storage.

It appears that the server is trying to run the "git apply" processes on different cores or cpus perhaps to try to spread the load on many of its cores. Anyway adding something like "taskset -c 7" in front of the "git rebase..." command, when launching it on the server, speeds it up, so that it takes around the same amount of time as it does on the laptop (6 seconds). "taskset -c 7" is just asking Linux to run a process and its children on core number 7, and it appears that doing that results in a much better cpu (or core) cache usage which explains the speed up.

If there was a config option called maybe "rebase.taskset" or "rebase.setcpuaffinity" that could be set to ask the OS for all the rebase child processes to be run on the same core, people who run many rebases on big repos on big servers as we do at Booking.com could easily benefit from a nice speed up.

Technically the option may make git-rebase--am.sh call "git am" using "taskset" (if taskset is available on the current OS).

Another possibility would be to libify the "git apply" functionality and then to use the libified "git apply" in run_apply() instead of launching a separate "git apply" process. One benefit from this is that we could probably get rid of the read_cache_from() call at the end of run_apply() and this would likely further speed up things. Also avoiding to launch separate processes might be a win especially on Windows.

Suggestions?

Thanks, Christian.