Re: [ublas] prod time - sparse matrix

From: Ian McCulloch (ianmcc_at_[hidden])
Date: 2006-08-16 17:26:38

Fred wrote:

> Hi,

[snip]

> I don't understand why prod spends "0 sec" in some iterations and
> "0.016sec" (in my computer) if variables are always the same...

Further to what Brian has already said, 0.016 is exactly 1/60, so that is
surely the resolution of the timer you are using.  You are timing just a
single 3x3 matrix-vector multiply, so almost all of the results will show 0
seconds, except a small number that just happen to cross a clock edge. 
i.e., 60 times a second the clock is incremented; the inner loop takes less
than 1/60 seconds to finish, but every so often the start and finish
straddle a clock edge.  (Aside: can you see from that how you could get an
estimate for the time based on the final value of 'counter' ? )

Move the timer out of the inner loop, and increase the number of iterations
until the time taken is relatively large (a second or so).

Cheers,
Ian