yes. the point is to compare the cost of implicitly calling `zero` (resulting in the equivalent of calling zero twice) to the cost of not initializing the memory before writing to it. I could alternatively have done: `@time x=zeros(); @time fill(x, 0)` to measure the same information.
On Mon Nov 24 2014 at 5:57:29 PM David Smith <[email protected]> wrote: > Did you mean to call zeros() in both cases? > > > On Monday, November 24, 2014 3:09:38 PM UTC-6, Jameson wrote: > >> It appears the fill operation accounts for about 0.15 seconds of the 6.15 >> seconds that my OS X laptop takes to create this array: >> >> $ ./julia -q >> >> *julia> **N=10^9* >> >> *1000000000* >> >> >> *julia> **@time begin x=zeros(Int64,N); fill(x,0) end* >> >> elapsed time: 6.325660691 seconds (8000136616 bytes allocated, 1.71% gc >> time) >> >> *0-element Array{Array{Int64,1},1}* >> >> >> $ ./julia -q >> >> *julia> **N=10^9* >> >> *1000000000* >> >> >> *julia> **@time x=zeros(Int64,N)* >> >> elapsed time: 6.160623835 seconds (8000014320 bytes allocated, 0.22% gc >> time) >> >> >> >> On Mon Nov 24 2014 at 3:18:39 PM Erik Schnetter <[email protected]> >> wrote: >> >>> On Mon, Nov 24, 2014 at 3:01 PM, David Smith <[email protected]> wrote: >>> > To add some data to this conversation, I just timed allocating a >>> billion >>> > Int64s on my macbook, and I got this (I ran these multiple times >>> before this >>> > and got similar timings): >>> > >>> > julia> N=1_000_000_000 >>> > 1000000000 >>> > >>> > julia> @time x = Array(Int64,N); >>> > elapsed time: 0.022577671 seconds (8000000128 bytes allocated) >>> > >>> > julia> @time x = zeros(Int64,N); >>> > elapsed time: 3.95432248 seconds (8000000152 bytes allocated) >>> > >>> > So we are talking adding possibly seconds to a program per large array >>> > allocation. >>> >>> This is not quite right -- the first does not actually map the pages >>> into memory; this is only done lazily when they are accessed the first >>> time. You need to compare "alloc uninitialized; then initialize once" >>> with "alloc zero-initialized; then initialize again". >>> >>> Current high-end system architectures have memory write speeds of ten >>> or twenty GByte per second; this is what you should see for very large >>> arrays -- this would be about 0.4 seconds for your case. For smaller >>> arrays, the data would reside in the cache, so that the allocation >>> overhead should be significantly smaller even. >>> >>> -erik >>> >>> -- >>> >> Erik Schnetter <[email protected]> >>> http://www.perimeterinstitute.ca/personal/eschnetter/ >>> >>
