Newsgroups: comp.parallel
From: Krste Asanovic <krste@icsi.berkeley.edu>
Subject: Re: Help - explain superlinear speedup?
Organization: International Computer Science Institute, Berkeley, CA, U.S.A.
Date: Fri, 13 Jan 1995 14:17:35 GMT
Message-ID: <3f46i7$cb2@agate.berkeley.edu>

In article <3f23ht$ec9@agate.berkeley.edu>, slater@nuc.berkeley.edu (Steve Slater) writes:
> 
> I have a program which has superlinear speedup and I
> can't explain it. Does anyone have any ideas. Here is
> the summary.
[deleted]

> With p4, I first run 4 processes (4 regions) on only 1 machine.
> The messages are passing through sockets. Then I run
> on 2 machines, each having 2 processes (2 regions), and
> finally on 4 machines, each having 1 process (region).
> 
> You would expect less than linear speedup since with
> only one machine, no messages are sent over the ethernet,
> they are just communicated via sockets. But I get very
> superlinear speedup like:
> 
> 1 proc:  556 sec               4 unique processes on 1 machine
> 2 proc:  204 sec               2 processes on each of 2 machines
> 4 proc:   38 sec               1 process on each machine
> 
> There was NO memory swapping occurring during the entire
> execution time. I would periodically check with ps.
> 
> Does anyone have any thoughts?

There are two possible cache effects. The first is that each Sparc-2
only has 64KB of unified cache. If your data set + code fits into 64KB
you'll see a marking improvement over the case when it doesn't.

The second is the limited TLB size. I don't have the Sparc-2 MMU
numbers handy, but I think it supported 64 entries for 4KB pages,
i.e. 256KB mapped simultaneously at most. If a single process's code +
data fits into the TLB, you'll see a marked difference.

These differences are exaggerated if your code makes repeated sweeps
over these data regions.

-- 
Krste Asanovic                                email: krste@icsi.berkeley.edu
International Computer Science Institute,     phone: +1 (510) 642-4274 x143
Suite 600, 1947 Center Street,                  fax: +1 (510) 643-7684
Berkeley, CA 94704-1198, USA                   http://www.icsi.berkeley.edu


