Here's the code to reproduce those issues:

import uvloop
from asyncio import set_event_loop_policy, create_subprocess_shell, gather, get_event_loop
from asyncio.subprocess import PIPE

async def run_cmd():
    length = 1000000
    process = await create_subprocess_shell("head -c %s /dev/urandom" % length, stdout=PIPE)
    output, err = await process.communicate()
    assert len(output) == length

set_event_loop_policy(uvloop.EventLoopPolicy())
loop = get_event_loop()
loop.set_debug(True)
runners = [run_cmd() for i in range(50)]
loop.run_until_complete(gather(*runners))

The exact runtime error raised depends on where I run it:

This raises multiple RuntimeError: Cannot pause_reading() when closing:

• uvloop version: 0.8.0
• Python version: 3.6.1
• Platform: linux 4.7.8-1 i686 pae, backport, Debian 8
• Can you reproduce the bug with PYTHONASYNCIODEBUG in env?: yes

On the same system with python 3.5.3 this runtime error isn't raised but the assertion fails.

This raises multiple exceptions such as RuntimeError: unable to perform operation on <ReadUnixTransport closed=True reading=False 0x7efe0d848d00>; the handler is closed when using /dev/zero instead of /dev/urandom and only needs 2 parallel subprocesses (1 works):

• uvloop version: 0.8.0
• Python version: 3.6.1
• Platform: linux 3.16.0-4-amd64, Debian 8
• Can you reproduce the bug with PYTHONASYNCIODEBUG in env?: yes

It also raises multiple times RuntimeError: Cannot pause_reading() when closing if run_cmd is launched 200 times instead of 50. Note that this system is on a server with 16G of RAM, while the first one only has 4G (if that makes any difference).

Running the same code with asyncio's default loop works in any case.