read-process-output-max vs system pipe capacity

[fredericgiquel]

The documentation suggests to set read-process-output-max to 1Mb to increase the amount of data read in one go from the language server. But the amount of data that actually transits also depends on system pipe capacity.

On GNU/Linux, the pipe size is limited to 16 pages (64kb) by default unless using fcntl to increase the pipe capacity (see pipe(7) manpage). But Emacs does not increase the pipe capacity so values of read-process-output-max above 64k are not taken into account. This behavior can be highlighted doing strace -e trace=write -p <pid_of_server> and triggering a request for a "big" completion or using scripts detailed in the following link : https://debbugs.gnu.org/cgi/bugreport.cgi?bug=55737
The patch that add the appropriate fcntl call has been merged in Emacs master branch last week (https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=a947c10d9093b9f3a29b60a52bda8afa43b6fd29).

I tested the system pipe capacity on VM with:

• FreeBSD: the pipe size is also limited to 64kb and there seems to be no way to increase it.
• Windows: there seems to be no enforced limit. I was able to read 20Mb in one go.

Not having access to the necessary hardware, I did not test on macOS. Can anyone test it?

[zikajk]

The pipe size is also limited to 64kb on macos. I used these scripts and dtruss to validate that.