Benchmarking Pants runs with multitime

Doing ... with PySpy

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.

One-time setup:

1. Ensure that gdb is installed.

• Ubuntu: sudo apt install gdb

2. 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 99-ptrace.conf with contents kernel.yama.ptrace_scope = 0. Note: This is a security exposure if you are not normally debugging processes across the process hierarchy.

3. Ensure that the debug info for your system Python binary is installed.

• Ubuntu: sudo apt install python3-dbg

Dumping thread stacks:

1. Find the pants binary (which may include pantsd if pantsd is enabled).

• Run: ps -ef | grep pants

2. Invoke gdb with the python binary and the process ID:

• Run: gdb /path/to/python/binary PROCESS_ID

3. Enable logging to write the thread dump to gdb.txt: set logging on

4. Dump all thread backtraces: thread apply all bt

5. 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)

6. Dump all Python stacks: thread apply all py-bt