perflock

Perflock is a tool that aids groups in sharing access to multi-user machines. It acts as a RWMutex, allowing either one process exclusive access, or multiple processes to that concurrent access.

Setup

Perflock uses a file lock to mediate access to the machine. You need to create the lock file /run/perflock before running perflock, and then set its permissions such that it is writeable by anyone who will run perflock.

Usage

To run an exclusive job (for example, a performance experiment that requires all cores):

$ perflock ./super --intensive job

If another job tries to run at the same time, it will join a queue:

$ perflock ../2nd/job --that -s big
Waiting for lock

When the first process finishes, the second one will be allowed to run.

Shared access

Some jobs are not performance critical, and simply want to not interfere with other, important jobs. Such jobs should be scheduled with pls (pronounced "please"). pls works the same way as perflock, but will allow multiple pls-wrapped jobs to execute in parallel, unless an exclusive job is currently running. If an exclusive job is later enqueued, it will wait until all the pls jobs have finished.

History

perflock was originally developed inside the Parallel and Distributed Operating Systems group at MIT CSAIL for use with our multi-core machines. Over the years, it has been rewritten and extended by many people in the group, to the point where it no longer can be said to "belong" to any of us. Once other groups asked how we share our machines fairly, we decided it might be useful to them, and so have decided to open-source it.

Mechanism

perflock is a very simple program -- it is really just a wrapper around flock, which provides exclusive locking for a UNIX file. We also include wholock, written by @aclements, which lists who is currently holding the lock, and what other jobs are waiting in the queue.