From e9db5ec0ba4f6b4426ba4cb47c49f25f79fe039c Mon Sep 17 00:00:00 2001 From: Ben Pfaff Date: Tue, 20 Dec 2005 20:42:15 +0000 Subject: [PATCH] Add summary of scheduler. --- doc/44bsd.texi | 35 ++++++++++++++++++++++++++++++++--- 1 file changed, 32 insertions(+), 3 deletions(-) diff --git a/doc/44bsd.texi b/doc/44bsd.texi index fc619c1..18c5a61 100644 --- a/doc/44bsd.texi +++ b/doc/44bsd.texi @@ -58,11 +58,14 @@ occur before any ordinary kernel thread has a chance to run, so that there is no chance that a kernel thread could see a newly increased @func{timer_ticks} value but old scheduler data values. +The 4.4@acronym{BSD} scheduler does not include priority donation. + @menu * Thread Niceness:: * Calculating Priority:: * Calculating recent_cpu:: * Calculating load_avg:: +* 4.4BSD Scheduler Summary:: * Fixed-Point Real Arithmetic:: @end menu @@ -225,9 +228,35 @@ Returns 100 times the current system load average, rounded to the nearest integer. @end deftypefun -@menu -* Fixed-Point Real Arithmetic:: -@end menu +@node 4.4BSD Scheduler Summary +@section Summary + +This section summarizes the calculations required to implement the +scheduler. It is not a complete description of scheduler requirements. + +Every thread has a @var{nice} value between -20 and 20 directly under +its control. Each thread also has a priority, between 0 +(@code{PRI_MIN}) through 63 (@code{PRI_MAX}), which is recalculated +using the following formula whenever the value of either term changes: + +@center @t{@var{priority} = (@var{recent_cpu} / 4) + (@var{nice} * 2)}. + +@var{recent_cpu} measures the amount of CPU time a thread has received +``recently.'' On each timer tick, the running thread's @var{recent_cpu} +is incremented by 1. Once per second, every thread's @var{recent_cpu} +is updated this way: + +@center @t{@var{recent_cpu} = (2*@var{load_avg})/(2*@var{load_avg} + 1) * @var{recent_cpu} + @var{nice}}. + +@var{load_avg} estimates the average number of threads ready to run over +the past minute. It is initialized to 0 at boot and recalculated once +per second as follows: + +@center @t{@var{load_avg} = (59/60)*@var{load_avg} + (1/60)*@var{ready_threads}}. + +@noindent where @var{ready_threads} is the number of threads that are +either running or ready to run at time of update (not including the idle +thread). @node Fixed-Point Real Arithmetic @section Fixed-Point Real Arithmetic -- 2.30.2