Python "BlockingIOError: [Errno 11] Resource temporarily unavailable"

A non-blocking I/O operation could not complete right now (EAGAIN/EWOULDBLOCK). Most often a subprocess produced output faster than it was drained and a non-blocking pipe filled - or the runner hit a thread/process limit.

What this error means

A program raises BlockingIOError: [Errno 11] Resource temporarily unavailable, frequently while writing to stdout/stderr, draining a subprocess pipe, or spawning threads. It can be intermittent and load-dependent.

Python traceback

BlockingIOError: [Errno 11] Resource temporarily unavailable
  File ".../subprocess.py", line ..., in _readerthread
# or on heavy logging to a non-blocking stdout pipe

Common causes

Full non-blocking pipe

A subprocess writes faster than the parent reads, and the pipe is non-blocking, so the write returns EAGAIN instead of blocking. Common when capturing a chatty child’s output by hand.

Thread or process resource limit

Hitting the max threads/processes (ulimit -u) or memory pressure can surface as EAGAIN when creating new OS resources.

How to fix it

Let subprocess drain output for you

Use communicate() (or run(..., capture_output=True)) instead of hand-rolling non-blocking reads from Popen pipes.

Python

out = subprocess.run(cmd, capture_output=True, text=True)
# or for streaming:
proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, text=True)
out, _ = proc.communicate()

Raise the process/thread limit

Terminal

ulimit -u 4096   # max user processes/threads
python run.py

How to prevent it

Drain subprocess output with communicate()/run(capture_output=True).
Bound thread/process pools rather than spawning unbounded workers.
When using non-blocking FDs, handle EAGAIN via select/poll instead of treating it as fatal.