In principle, it’s also best to wrap all of this in a function, although it doesn’t seem to matter that much for this case (on my machine).
I get little over 0.6 seconds for the first, and about 0.55 s for the second and third. That sounds consistent with my expectation. Note also that the statement with rand has a slightly higher allocation. Since matrix multiplication seems to be slower on your machines (less cores?), whereas the time of rand is probably similar (since it is not multithreaded anyway if I am correct), then I guess the effect of rand is just unobservable in your timings. On 17 Jul 2014, at 15:54, Tomas Lycken <tomas.lyc...@gmail.com> wrote: > @Jutho: My gut reaction was the same thing, but then I should be able to > reproduce the results, right? All three invocations take about 1.2-1.5 > seconds on my machine. > > // T > > On Thursday, July 17, 2014 3:06:08 PM UTC+2, Jutho wrote: > I don't know about the zeros, but one issue with your timings is certainly > that you also measure the time to generate the random numbers, which is most > probably not negligible. > > Op donderdag 17 juli 2014 13:54:54 UTC+2 schreef Andrei Zh: > I continue investigating matrix multiplication performance. Today I found > that multiplication by array of zeros(..) is several times faster than > multiplication by array of ones(..) or random numbers: > > julia> A = rand(200, 100) > ... > > julia> @time for i=1:1000 A * rand(100, 200) end > elapsed time: 3.009730414 seconds (480160000 bytes allocated, 11.21% gc time) > > julia> @time for i=1:1000 A * ones(100, 200) end > elapsed time: 2.973320655 seconds (480128000 bytes allocated, 12.72% gc time) > > julia> @time for i=1:1000 A * zeros(100, 200) end > elapsed time: 0.438900132 seconds (480128000 bytes allocated, 85.46% gc time) > > So, A * zeros() is about 6 faster than other kinds of multiplication. Note > also that it uses ~7x more GC time. > > On NumPy no such difference is seen: > > In [106]: %timeit dot(A, rand(100, 200)) > 100 loops, best of 3: 2.77 ms per loop > > In [107]: %timeit dot(A, ones((100, 200))) > 100 loops, best of 3: 2.59 ms per loop > > In [108]: %timeit dot(A, zeros((100, 200))) > 100 loops, best of 3: 2.57 ms per loop > > > So I'm curious, how multiplying by zeros matrix is different from other > multiplication types? > >
