Debugging and benchmarking
Some techniques to figure out why Pants is behaving the way it is.
hyperfine to benchmark, especially comparing before and after to see the impact of a change: https://github.com/sharkdp/hyperfine.
When benchmarking, you must decide if you care about cold cache performance vs. warm cache (or both). If cold, use
--no-pantsd --no-process-execution-local-cache. If warm, use hyperfine's option
❯ hyperfine --warmup=1 --runs=5 './pants list ::` ❯ hyperfine --runs=5 './pants --no-pantsd --no-process-execution-local-cache lint ::'
Profiling with PySpy (TODO)
Identifying the impact of Python's GIL (on macOS)
Obtaining Full Thread Backtraces
Pants runs as a Python program that calls into a native Rust library. In debugging locking and deadlock issues, it is useful to capture dumps of the thread stacks in order to figure out where a deadlock may be occurring.
- Ensure that gdb is installed.
sudo apt install gdb
- Ensure that the kernel is configured to allow debuggers to attach to processes that are not in the same parent/child process hierarchy.
echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
- To make the change permanent, add a file to /etc/sysctl.d named
kernel.yama.ptrace_scope = 0. Note: This is a security exposure if you are not normally debugging processes across the process hierarchy.
- Ensure that the debug info for your system Python binary is installed.
sudo apt install python3-dbg
Dumping thread stacks:
- Find the pants binary (which may include pantsd if pantsd is enabled).
ps -ef | grep pants
- Invoke gdb with the python binary and the process ID:
gdb /path/to/python/binary PROCESS_ID
- Enable logging to write the thread dump to
set logging on
- Dump all thread backtraces:
thread apply all bt
- If you use pyenv to mange your Python install, a gdb script will exist in the same directory as the Python binary. Source it into gdb:
source ~/.pyenv/versions/3.8.5/bin/python3.8-gdb.py(if using version 3.8.5)
- Dump all Python stacks:
thread apply all py-bt
Updated over 1 year ago