Slow load time of bash in cygwin

Question

At the moment bash takes about 2 seconds to load. I have ran bash with -x flag and I am seeing the output and it seems as though PATH is being loaded many times in cygwin. The funny thing is I use the same file in linux environment, but it works fine, without the reload problem. Could the following cause the problem?

if [ `uname -o` = "Cygwin" ]; then
    ....
fi

Answer 1

My answer is the same as npe's above. But, since I just joined, I cannot comment or even upvote it! I hope this post doesn't get deleted because it offer reassurance for anyone looking for an answer to the same problem.

npe's solution worked for me. There's only one caveat - I had to close all cygwin processes before I got the best out of it. That includes running cygwin services, like sshd, and the ssh-agent that I start from my login scripts. Before that, the window for the cygwin terminal would appear instantly but hang for several seconds before it presents the prompt. And it hanged for several seconds upon closing the window. After I killed all processes and started the cygserver service (btw I prefer to use the Cygwin way - 'cygrunsrv -S cygserver', than 'net start cygserver'; I don't know if it makes any practical difference) it starts immediately. So thanks to npe again!

Answer 2

I'm on a corporate network with a pretty complicated setup, and it seems that really kills cygwin startup times. Related to npe's answer, I also had to follow some of the steps laid out here: https://cygwin.com/faq/faq.html#faq.using.startup-slow

Another cause for AD client system is slow DC replies, commonly observed in configurations with remote DC access. The Cygwin DLL queries information about every group you're in to populate the local cache on startup. You may speed up this process a little by caching your own information in local files. Run these commands in a Cygwin terminal with write access to /etc:

getent passwd $(id -u) > /etc/passwd getent group $(id -G) > /etc/group

Also, set /etc/nsswitch.conf as follows:

passwd: files db group: files db

This will limit the need for Cygwin to contact the AD domain controller (DC) while still allowing for additional information to be retrieved from DC, such as when listing remote directories.

After doing that plus starting the cygserver my cygwin startup time dropped significantly.

Answer 3

All of the answers refer to older versions of bash_completion, and are irrelevant for recent bash_completion.

Modern bash_completion moved most of the completion files to /usr/share/bash-completion/completions by default, check the path on your system by running

# pkg-config --variable=completionsdir bash-completion
/usr/share/bash-completion/completions

There are many files in there, one for each command, but that is not a problem, since they are loaded on demand the first time you use completion with each command. The old /etc/bash_completion.d is still supported for compatibility, and all files from there are loaded when bash_completion starts.

# pkg-config --variable=compatdir bash-completion
/etc/bash_completion.d

Use this script to check if there are any stale files left in the old dir.

#!/bin/sh
COMPLETIONS_DIR="$(pkg-config --variable=completionsdir bash-completion)"
COMPAT_DIR="$(pkg-config --variable=compatdir bash-completion)"
for file in "${COMPLETIONS_DIR}"/*; do
    file="${COMPAT_DIR}/${file#${COMPLETIONS_DIR}/}"
    [ -f "$file" ] && printf '%s\n' $file
done

It prints the list of files in compat dir that are also present in the newer (on-demand) completions dir. Unless you have specific reasons to keep some of them, review, backup and remove all of those files.

As a result, the compat dir should be mostly empty.

Now, for the most interesting part - checking why bash startup is slow. If you just run bash, it will start a non-login, interactive shell - this one on Cygwin source /etc/bash.bashrc and then ~/.bashrc. This most likely doesn't include bash completion, unless you source it from one of rc files. If you run bash -l (bash --login), start Cygwin Terminal (depends on your cygwin.bat), or log in via SSH, it will start a login, interactive shell - which will source /etc/profile, ~/.bash_profile, and the aforementioned rc files. The /etc/profile script itself sources all executable .sh files in /etc/profile.d.

You can check how long each file takes to source. Find this code in /etc/profile:

for file in /etc/profile.d/*.$1; do
  [ -e "${file}" ] && . "${file}"
done

Back it up, then replace it with this:

for file in /etc/profile.d/*.$1; do
  TIMEFORMAT="%3lR ${file}"
  [ -e "${file}" ] && time . "${file}"
done

Start bash and you will see how long each file took. Investigate files that take a significant amount of time. In my case, it was bash_completion.sh and fzf.sh (fzf is fuzzy finder, a really nice complement to bash_completion). Now the choice is to disable it or investigate further. Since I wanted to keep using fzf shortcuts in bash, I investigated, found the source of the slowdown, optimized it, and submitted my patch to fzf's repo (hopefully it will be accepted).

Now for the biggest time spender - bash_completion.sh. Basically that script sources /usr/share/bash-completion/bash_completion. I backed up that file, then edited it. On the last page there is for loop that sources all the files in compat dir - /etc/bash_completion.d. Again, I added TIMEFORMAT and time, and saw which script was causing the slow starting. It was zzz-fzf (fzf package). I investigated and found a subshell ($()) being executed multiple times in a for loop, rewrote that part without using a subshell, making the script work quickly. I already submitted my patch to fzf's repo.

The biggest reason for all these slowdowns is: fork is not supported by Windows process model, Cygwin did a great job emulating it, but it's painfully slow compared to a real UNIX. A subshell or a pipeline that does very little work by itself spends most of it's execution time for fork-ing. E.g. compare the execution times of time echo msg (0.000s on my Cygwin) vs time echo $(echo msg) (0.042s on my Cygwin) - day and night. The echo command itself takes no appreciable time to execute, but creating a subshell is very expensive. On my Linux system, these commands take 0.000s and 0.001s respectively. Many packages Cygwin has are developed by people who use Linux or other UNIX, and can run on Cygwin unmodified. So naturally these devs feel free to use subshells, pipelines and other features wherever convenient, since they don't feel any significant performance hit on their system, but on Cygwin those shell scripts might run tens and hundreds of times slower.

Bottom line, if a shell script works slowly in Cygwin - try to locate the source of fork calls and rewrite the script to eliminate them as much as possible. E.g. cmd="$(printf "$1" "$2")" (uses one fork for subshell) can be replaced with printf -v cmd "$1" "$2".

Boy, it came out really long. Any people still reading up to here are real heros. Thanks :)

Answer 4

As someone mentioned above, one possible issue is the PATH environment variable contains too much path, cygwin will search all of them. I prefer direct edit the /etc/profile, just overwrite the PATH variable to cygwin related path, e.g. PATH="/usr/local/bin:/usr/bin". Add additional path if you want.