4.4BSD timeslice management

John Baldwin jhb at freebsd.org
Tue Oct 11 12:34:16 UTC 2011

On Wednesday, October 05, 2011 12:54:57 pm Riccardo Cattaneo wrote:
> Hello,
> I'm looking into the code of 4.4BSD scheduler (and related files) of FreeBSD 
version 7.2-RELEASE on an amd64 machine in order to understand how to manage 
time slices. I'm trying to dynamically change the length of the time slice on 
a thread by thread basis to "give more priority" or "less priority" to 
> In function sched_clock (sched_4bsd.c) I found a piece of code in which 
thread's flags are added with TDF_NEEDRESCHED when the time slice (quantum) 
expires but I failed to notice any point in the code that actually preempt the 
> Adding a per-thread parameter (e.g., in struct thread) I could set 
TDF_NEEDRESCHED when the dynamic time slice expires but how can I be sure the 
thread gets preempted? Where can I set a per-thread parameter (maybe after 
cpu_switch?!)? Is there a timer periodically calling a sort of context switch 
function or the context switches happen only when triggered by the kernel 

TDF_NEEDRESCHED is checked for in ast() in sys/kern/subr_trap.c.  It is called 
during return to userland from interrupts, traps, and system calls.

John Baldwin

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