We're experiencing an issue where occasionally (perhaps on bad commits), the git diff commands that Phabricator runs during background processing thrash the disk I/O and CPU, and basically prevent the server from doing any other work while that's occurring. For example, this is the New Relic graph of server activity when this occurs:
To prevent this, I'd like to be able to set the I/O nicety level of the processes Phabricator spawns to be lower than normal; i.e. I'd like Phabricator to run ionice 4 against the Git processes it spawns. While this will make Git diff processing take longer, it should ensure that other processes have access to disk I/O first, and hopefully not kill the server from serving all traffic when it occurs.
I didn't file this as a bug report, because I don't have reproductions steps (it seems to occur at random, but running kill -KILL $(pidof git) fixes it).