Message-ID: <i2h57e838141005040555p9ba373fbkb07711e476f0c24@mail.gmail.com>
Date: 2010-05-04T12:55:11Z
From: Stephen Weston
Subject: How to check if %dopar% really run parallel?
In-Reply-To: <4BDFA11B.3000809@cscs.ch>
There is a mistake. Rather than:
times(10000) %dopar% fun
you should write:
times(10000) %dopar% fun()
On my machine, "fun" executes in about 0.4 seconds, so executing
it 10,000 times should take over an hour to execute. Your error turned
a real program into a toy program. The error also resulted in more
communication, since now the function itself is being returned by the
workers.
When I ran your benchmark on my machine with 100, rather than 10,000
tasks, I got the following results:
user system elapsed
43.573 0.191 43.823
user system elapsed
0.093 0.007 24.890
That's not so bad.
- Steve
On Tue, May 4, 2010 at 12:22 AM, Mario Valle <mvalle at cscs.ch> wrote:
> Is there any way to check that %dopar% really runs parallel?
> The following code (on a dual core laptop running windows+R 2.11.0pat and on
> Linux+R2.11.0) runs %dopar% more slowly than the same %do% code.
> BTW, if you see any obvious mistake in the code...
> Thanks!
> ? ? ? ? ? ? ? ?mario
>
>
> library(doSNOW)
> library(foreach)
>
> fun <- function() for(q in 1:1000000) sqrt(3)
>
> system.time(times(10000) %do% fun, gcFirst = TRUE)
> # ? user ?system elapsed
> # ? 5.74 ? ?0.01 ? ?6.24
>
> cl <- makeCluster(2, type = "SOCK")
> registerDoSNOW(cl)
>
> system.time(times(10000) %dopar% fun, gcFirst = TRUE)
> # ? user ?system elapsed
> # ? 7.89 ? ?0.19 ? ?9.01
>
> stopCluster(cl)
>
> --
> Ing. Mario Valle
> Data Analysis and Visualization Group ? ? ? ? ? ?|
> http://www.cscs.ch/~mvalle
> Swiss National Supercomputing Centre (CSCS) ? ? ?| Tel: ?+41 (91) 610.82.60
> v. Cantonale Galleria 2, 6928 Manno, Switzerland | Fax: ?+41 (91) 610.82.82
>
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