Hi there, I'm writing a multithreaded merge sort that suffers from massive amounts of context switches (as seen by vmstat). The outline of the program is roughly as follows:
====
my @sortdata : shared; // the values to be sorted
[...]
// starting two threads
new threads( \&mergesort_string, $begin, $half, $threaddepth - 1 );
new threads( \&mergesort_string, $half, $end, $threaddepth - 1 );
[...]
sub mergesort_string {
my ( $begin, $end, $threaddepth ) = @_;
// sorting "my" part of @sortdata, as delimited
// by $begin and $end
}
====
By playing around, I discovered that the "shared" is what causes the
context switches. They take up about 40% of the duration of the program
for an array of 10,000 elements.
I don't need any locking on the elements itself because the merge sort
algorithm itself guarantees in my opinion that there is no conflicting
access to them.
Am I missing something here? How can I avoid those context switches that
make the program essentially useless?
Thanks!
Thomas
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