Git operations slow compared to WSL2 on same machine

[HenryGessau]

If I clone a repo in the provided bash shell in Windows 11:

$ time __git_ps1
 (master|u=)
real    0m0.574s
user    0m0.000s
sys     0m0.030s

If I clone the same repo in WSL2 (Ubuntu) on the same machine:

$ time __git_ps1
 (master|u=)
real    0m0.032s
user    0m0.024s
sys     0m0.009s

Why is the git performance so much slower in the git-for-windows environment?

[dscho]

The problem is Git's over-reliance on multi-process operations. Basically, whenever you call git.exe, you pay a comparatively minor price on Linux because its fork() syscall is pretty quick, while the equivalent CreateProcess() takes much longer.

It gets even worse when you do that in Git Bash, as the __git_ps1 Bash function does: In this environment, the processes are actually created via a fork() syscall emulated by the POSIX emulation layer (also known as "the MSYS2 runtime", which is a friendly fork of the Cygwin runtime). This not only pays the price of a CreateProcess() call, it actually pays it twice because Unix-style signal handling is emulated for non-MSYS2 processes (such as git.exe) via a second process, and to add insult to injury, the fork() syscall has to copy the memory from the parent process and re-open file handles to imitate what fork() does on Unix. This is particularly painful because the only thing Bash does right after the POSIX emulation went through all of those steps _is to toss all of that away by calling exec() and replacing everything with the new git.exe process.

You can see all of those processes being spawned if you call set -x and after that simply hit Enter.

So yeah, it's quite slow, and it's a shame.

[HenryGessau]

Thanks for the detailed explanation.