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NNTP-Posting-Date: Thu, 15 Sep 2005 00:10:43 -0500
From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: puzzled re hyperthreaded performance
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Date: Thu, 15 Sep 2005 00:10:43 -0500
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Date: 2005-09-15T00:10:43-05:00
List-Id: <comp.lang.ada>

  I have a multitasking program that runs faster if I insert a "delay d;"
statement to prevent some of the multitasking overlap.
  This is a version of the Shootout k-nucleotide program, which involves
a series of create/fill/lookup in hash tables, using the Ada 95 version
of Ada.Containers.Indefinite_Hashed_Maps  I let the operations run in
two tasks, each with their own hash table.  Memory requirement are minor.
    Alt_Task.Start;
    delay d;
    Compute;
    Alt_Task.Finish;
Alt_Task performs the same computations as Compute, reading the same
String, but sharing no other data.  There are no obvious task interactions,
and I don't see any in Ada.Containers
  For every additional 1.0 seconds of "d", the sequence runs about 1.5
seconds faster.  This is Gnat 3.15p on W2k on a hyperthreaded Pentium
(two virtual CPUs).
  Any suggestions what's going on?