Different thread behavior depending on condition - help!

Question

Hi,
I have a multithreaded app in which, if certain certain predicate is met, then the current thread will execute some code before and after every invocation to a list:
foo() {
   //some code...
   if (predicate)
     do_stuff_before
     list.X
     do_stuff_after
  else
     list.X
  //some code...
}However, the evaluation of predicate() is very expensive. Therefore, I was wondering whether there is some way of dynamically changing foo such that
a) if the predicate has been set to false for the current thread, it executes
foo() {
//some code...
list.X
// some code...
}b) if the predicate has been set to true for the current thread, it executes
foo() {
//some code...
do_stuff_before
list.X
do_stuff_after
// some code...
}i.e., having two versions of the same code. Some threads execute version (a), while others execute version (b). Is this possible? How? Thanks!
Edited by: Leirado on Sep 11, 2009 9:15 AM
Edited by: Leirado on Sep 11, 2009 9:17 AM

Answer 1

You haven't made it clear, but I'm assuming that the value of the predicate is determined by various complex, expensive to evaluate conditions occurring independently of the worker threads. That is, the value of the predicate can change at any point before, during, or after execution of your before/list.X/after code.
Given that, I'd suggest on thread that simply loops to evaluate the predicate, and set s boolean flag as soon as it knows the truth or falsehood thereof. The worker threads read that flag for your if/else.
Make sure to either make the flag volatile, or synchronize all access to it.
TP: True predicate. This is whatever conditions external to the worker
and predicate eval threads are occurring that determine the truth of the
predicate.
PET: Predicate eval thread. Simply loops repeatedly to evaluate
the predicate, and at the end of each evaluation, sets a boolean flag
representing the latest value.
WT: Worker threads. They check the flag set by PET for the if/else
described in your first post.
TIME----------------------------------------------------------->
   init to              becomes true                           
TP-false--------------------+----------------------------------
                            |                                  
                           here                                
                                            now finds it true
     pred found  expensive code looping      |        
PET--initially---and taking much time to-----+-----------------
     false       evaluate full predicate...  |
                                           and sets flag
     flag==false     flag==false              flag==true
WT1--exec else--------exec else-----------------exec if-------------
                                 flag==false              flag==true
WT2-------------------------------exec else----------------exec if--