> STINNER Victor added the comment:
>
> Since Popen.communicate() returns the whole content of the buffer, would it be safe to increase the buffer size? For example, use 4 GB as the buffer size?

Sure, if you want to pay the CPU and memory overhead of allocating a
4GB buffer :-)

> Should communicate() be "fair" between stdout and stderr?

There's no reason to make the buffer depend on the FD.

> To choose a new value, we need benchmark results on different OSes, at least Windows, Linux, FreeBSD (and maybe also Solaris) since these 3 OSes have a different kernel.

Windows isn't concerned by this, since it doesn't use a selector, but threads.
For the other OSes, that's why I opened this issue (you forgot AIX in
your list :-).