sched_4BSD

[Julian Elischer]

Ashwin Chandra wrote:
> I wanted to get some clarification about the 4BSD scheduler. I am sort of
> confused why there are two forms of scheduling, one done between processes and
> another done between threads in a process. The priority calculations seem to be
> done only with processes and I assume that the global run queue holds processes,
> not threads. Also why is there only 1 run queue for 1 CPU. What happens to
> blocked processes and ready to be runned processes?

Part of the challenge of adding threads to a system is to make it hard for a 
threaded process to "flood" the system run queues so that other processes
get no cpu time.

The scheme in the current freeBSD schedulers is a "crude" method, by which
only a limitted number of threads per process are allowed to be added to
the system run queue. RUnnable hreads fo r aprocess are kept on a run queue for 
the process and only the highest N prioriy  hreads are actually put on the
system run queue.

This is by no means the best way, but rather the
easiest way. I am hoping that some PhD candidate somewhere will decide
that thread scheduling is his topic and will figure out a better way
of doing this.

both run queues hold threads. This is still a place wjere a lot
of work can be done.

:-)


> 
> Ash
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