Add hints about not doing too much work in timer interrupt.

diff --git a/doc/threads.texi b/doc/threads.texi
index ac28ba86830da78c8431fd400edd164c5d959b94..1231168e34224ab3aca2ee50479087ceb0abdf46 100644 (file)
--- a/doc/threads.texi
+++ b/doc/threads.texi
@@ -791,4 +791,32 @@ scheduler at the same time.
 
 Yes.  In general, your implementation may differ from the description,
 as long as its behavior is the same.
 
 Yes.  In general, your implementation may differ from the description,
 as long as its behavior is the same.

+
+@item Some scheduler tests fail and I don't understand why.  Help!
+
+If your implementation mysteriously fails some of the advanced
+scheduler tests, try the following:
+
+@itemize
+@item
+Read the source files for the tests that you're failing, to make sure
+that you understand what's going on.  Each one has a comment at the
+top that explains its purpose and expected results.
+
+@item
+Double-check your fixed-point arithmetic routines and your use of them
+in the scheduler routines.
+
+@item
+Consider how much work your implementation does in the timer
+interrupt.  If the timer interrupt handler takes too long, then it
+will take away most of a timer tick from the thread that the timer
+interrupt preempted.  When it returns control to that thread, it
+therefore won't get to do much work before the next timer interrupt
+arrives.  That thread will therefore get blamed for a lot more CPU
+time than it actually got a chance to use.  This raises the
+interrupted thread's recent CPU count, thereby lowering its priority.
+It can cause scheduling decisions to change.  It also raises the load
+average.
+@end itemize

 @end table
 @end table

diff --git a/src/tests/threads/mlfqs-load-avg.c b/src/tests/threads/mlfqs-load-avg.c
index 8ea4dd14a9243638f16dec6ffefde2b753a1f6a1..50e83e276406714b03ab8d199a16259ee574ffbf 100644 (file)
--- a/src/tests/threads/mlfqs-load-avg.c
+++ b/src/tests/threads/mlfqs-load-avg.c
@@ -4,7 +4,17 @@
    seconds, starting 10 seconds in, the main thread prints the
    load average.
 
    seconds, starting 10 seconds in, the main thread prints the
    load average.
 

-   The expected output is this (some margin of error is allowed):
+   The expected output is listed below.  Some margin of error is
+   allowed.
+
+   If your implementation fails this test but passes most other
+   tests, then consider whether you are doing too much work in
+   the timer interrupt.  If the timer interrupt handler takes too
+   long, then the test's main thread will not have enough time to
+   do its own work (printing a message) and go back to sleep
+   before the next tick arrives.  Then the main thread will be
+   ready, instead of sleeping, when the tick arrives,
+   artificially driving up the load average.

 
    After 0 seconds, load average=0.00.
    After 2 seconds, load average=0.05.
 
    After 0 seconds, load average=0.00.
    After 2 seconds, load average=0.05.