Decreasing the Control Overhead of the Unlimited Speculative Execution

Hayato YAMANA   Mitsuhisa SATO   Yuetsu KODAMA   Hirohumi SAKANE 
  Shuichi SAKAI*   Yoshinori YAMAGUCHI
Electrotechnical Laboratory        *Real World Computing Partnarship
1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan

 This paper discusses how to decrease the control overhead of tasks with 
speculation on multiprocessors. Firstly, we have implemented the unlimited 
speculative execution on the EM-4 multiprocessor. Secondly, the overhead 
is classified into its several sources.  After measuring each classified 
overhead, it has been confirmed that both the broadcast latency and the 
overhead initiating tasks are not major factors. Insted, the overhead of 
receiving and manipulating broadcasted control data is major factor. When 
the factor is decreased by 1/4, the speedup ratio increases up to 3 and 
we will have 10 times speedup on 32 PEs with the program whose theoretical
 speedup ratio is 13.6.