X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=doc%2F44bsd.texi;h=743dad36fca230323600e4f9add0465fcad742df;hb=919347c164606c3f1544b2e8bd62f505aeda80a1;hp=e53e829ce2be438f9d1621eb5f8e1c94f891f715;hpb=be9aa9a652ea92ba5459c9337b6738de61b54eae;p=pintos-anon

diff --git a/doc/44bsd.texi b/doc/44bsd.texi
index e53e829..743dad3 100644
--- a/doc/44bsd.texi
+++ b/doc/44bsd.texi
@@ -1,4 +1,4 @@
-@node 4.4BSD Scheduler, Coding Standards, References, Top
+@node 4.4BSD Scheduler
 @appendix 4.4@acronym{BSD} Scheduler
 
 @iftex
@@ -44,7 +44,7 @@ vary over time.  A well-designed scheduler can often accommodate threads
 with all these requirements simultaneously.
 
 For project 1, you must implement the scheduler described in this
-appendix.  Our scheduler resembles the one described in @bibref{4.4BSD},
+appendix.  Our scheduler resembles the one described in @bibref{McKusick},
 which is one example of a @dfn{multilevel feedback queue} scheduler.
 This type of scheduler maintains several queues of ready-to-run threads,
 where each queue holds threads with a different priority.  At any given
@@ -76,17 +76,16 @@ Thread priority is dynamically determined by the scheduler using a
 formula given below.  However, each thread also has an integer
 @dfn{nice} value that determines how ``nice'' the thread should be to
 other threads.  A @var{nice} of zero does not affect thread priority.  A
-positive @var{nice}, to the maximum of 20, increases the numeric
-priority of a thread, decreasing its effective priority, and causes it
-to give up some CPU time it would otherwise receive.  On the other hand,
-a negative @var{nice}, to the minimum of -20, tends to take away CPU
-time from other threads.
+positive @var{nice}, to the maximum of 20, decreases the priority of a 
+thread and causes it to give up some CPU time it would otherwise receive.
+On the other hand, a negative @var{nice}, to the minimum of -20, tends
+to take away CPU time from other threads.
 
 The initial thread starts with a @var{nice} value of zero.  Other
 threads start with a @var{nice} value inherited from their parent
 thread.  You must implement the functions described below, which are for
 use by test programs.  We have provided skeleton definitions for them in
-@file{threads/thread.c}.  by test programs
+@file{threads/thread.c}.
 
 @deftypefun int thread_get_nice (void)
 Returns the current thread's @var{nice} value.
@@ -104,17 +103,19 @@ highest priority, yields.
 
 Our scheduler has 64 priorities and thus 64 ready queues, numbered 0
 (@code{PRI_MIN}) through 63 (@code{PRI_MAX}).  Lower numbers correspond
-to @emph{higher} priorities, so that priority 0 is the highest priority
-and priority 63 is the lowest.  Thread priority is calculated initially
+to lower priorities, so that priority 0 is the lowest priority
+and priority 63 is the highest.  Thread priority is calculated initially
 at thread initialization.  It is also recalculated once every fourth
 clock tick, for every thread.  In either case, it is determined by
 the formula
 
-@center @t{@var{priority} = (@var{recent_cpu} / 4) + (@var{nice} * 2)},
+@center @t{@var{priority} = @code{PRI_MAX} - (@var{recent_cpu} / 4) - (@var{nice} * 2)},
 
 @noindent where @var{recent_cpu} is an estimate of the CPU time the
 thread has used recently (see below) and @var{nice} is the thread's
-@var{nice} value.  The coefficients @math{1/4} and 2 on @var{recent_cpu}
+@var{nice} value.  The result should be rounded down to the nearest
+integer (truncated).
+The coefficients @math{1/4} and 2 on @var{recent_cpu}
 and @var{nice}, respectively, have been found to work well in practice
 but lack deeper meaning.  The calculated @var{priority} is always
 adjusted to lie in the valid range @code{PRI_MIN} to @code{PRI_MAX}.
@@ -160,12 +161,13 @@ weight of @math{a} at time @math{t+1}, @am{a^2, a**2} at time
 @math{f(t)} has a weight of approximately @math{1/e} at time @math{t+k},
 approximately @am{1/e^2, 1/e**2} at time @am{t+2k, t+2*k}, and so on.
 From the opposite direction, @math{f(t)} decays to weight @math{w} at
-@am{t = \log_aw, t = ln(w)/ln(a)}.
+time @am{t + \log_aw, t + ln(w)/ln(a)}.
 
 The initial value of @var{recent_cpu} is 0 in the first thread
 created, or the parent's value in other new threads.  Each time a timer
 interrupt occurs, @var{recent_cpu} is incremented by 1 for the running
-thread only.  In addition, once per second the value of @var{recent_cpu}
+thread only, unless the idle thread is running.  In addition, once per
+second the value of @var{recent_cpu}
 is recalculated for every thread (whether running, ready, or blocked),
 using this formula:
 
@@ -175,14 +177,14 @@ using this formula:
 threads ready to run (see below).  If @var{load_avg} is 1, indicating
 that a single thread, on average, is competing for the CPU, then the
 current value of @var{recent_cpu} decays to a weight of .1 in
-@am{\log_{2/3}.1 \approx 6, ln(2/3)/ln(.1) = approx. 6} seconds; if
+@am{\log_{2/3}.1 \approx 6, ln(.1)/ln(2/3) = approx. 6} seconds; if
 @var{load_avg} is 2, then decay to a weight of .1 takes @am{\log_{3/4}.1
-\approx 8, ln(3/4)/ln(.1) = approx. 8} seconds.  The effect is that
+\approx 8, ln(.1)/ln(3/4) = approx. 8} seconds.  The effect is that
 @var{recent_cpu} estimates the amount of CPU time the thread has
 received ``recently,'' with the rate of decay inversely proportional to
 the number of threads competing for the CPU.
 
-Assumptions made by some of the tests require that updates to
+Assumptions made by some of the tests require that these recalculations of
 @var{recent_cpu} be made exactly when the system tick counter reaches a
 multiple of a second, that is, when @code{timer_ticks () % TIMER_FREQ ==
 0}, and not at any other time.
@@ -190,6 +192,11 @@ multiple of a second, that is, when @code{timer_ticks () % TIMER_FREQ ==
 The value of @var{recent_cpu} can be negative for a thread with a
 negative @var{nice} value.  Do not clamp negative @var{recent_cpu} to 0.
 
+You may need to think about the order of calculations in this formula.
+We recommend computing the coefficient of @var{recent_cpu} first, then
+multiplying.  Some students have reported that multiplying
+@var{load_avg} by @var{recent_cpu} directly can cause overflow.
+
 You must implement @func{thread_get_recent_cpu}, for which there is a
 skeleton in @file{threads/thread.c}.
 
@@ -231,15 +238,15 @@ nearest integer.
 @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.
+The following formulas summarize the calculations required to implement the
+scheduler.  They are 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:
+using the following formula every fourth tick:
 
-@center @t{@var{priority} = (@var{recent_cpu} / 4) + (@var{nice} * 2)}.
+@center @t{@var{priority} = @code{PRI_MAX} - (@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}
@@ -293,8 +300,8 @@ Suppose that we are using a @m{p.q} fixed-point format, and let @am{f =
 2^q, f = 2**q}.  By the definition above, we can convert an integer or
 real number into @m{p.q} format by multiplying with @m{f}.  For example,
 in 17.14 format the fraction 59/60 used in the calculation of
-@var{load_avg}, above, is @am{(59/60)2^{14}, 59/60*(2**14)} = 16,111
-(rounded to nearest).  To convert a fixed-point value back to an
+@var{load_avg}, above, is @am{(59/60)2^{14}, 59/60*(2**14)} = 16,110.
+To convert a fixed-point value back to an
 integer, divide by @m{f}.  (The normal @samp{/} operator in C rounds
 toward zero, that is, it rounds positive numbers down and negative
 numbers up.  To round to nearest, add @m{f / 2} to a positive number, or