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

[pintos-anon] / doc / threads.texi

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