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]
> <javascript:>> wrote:
>
>> On Mon, Nov 24, 2014 at 3:01 PM, David Smith <[email protected]
>> <javascript:>> 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] <javascript:>>
>> http://www.perimeterinstitute.ca/personal/eschnetter/
>>
>
