Newsgroups: comp.parallel.pvm
From: Ki YangSuk <enigma@comp.snu.ac.kr>
Subject: Bad Performance on SP2?
Organization: Seoul National Univ. Computing Center
Date: Thu, 18 Apr 1996 13:35:07 +0900
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Message-ID: <3175C67B.41C67EA6@comp.snu.ac.kr>

First, I thank you for your reading my question.
I converted serial MPEG2 encoder to parallel one using pvm library on
SP2.
Desparately, I found that the performance of parallel MPEG2 encoder is
worse than that of serial one.
I run parallel MPEG2 program under subscribed condition.

1. All nodes needed are reserved and communications are done through
high performance switch.
2. The program is SPMD style and one task named parent does I/O and the
others named child encode. So, parent read frame from file and
distributes sub frame after dividing the frame accoding to processor
number.
3. I run this program on two node, one for parent, the other for child.
So, whole input frames are encoded by one child like serial version.

In this condition, total run time for parallel version is longer than
serial vertion because of communication overhead, but real computation
time - the time for common codes for two version except communication
time - for encoding must be little different.
But result is pessimistic. Parallel version is three or four times
slower than serial version.
I don't know what is the matter.
Why is the program slower on the same data and code.

