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Possible bug: very slow module load after addprocs()
7 posts by 2 authors  



Marius Millea 	

Jul 19


I noticed that once I addprocs(), subsequent "using" statements were extremely slow. I guess in this case its loading the module on each processor, but if it happens in parallel it shouldn't be *that* much more wall time, and here I'm talking about two orders of magnitude difference. 

Assuming I've got a file Empty.jl who contents is,

module Empty
end

then single threaded:

tic()
using Empty
toc()
elapsed time: 0.024461076 seconds

vs. multi-threaded:

addprocs() #I've got 8 procs
tic()
using Empty
toc()
elapsed time: 2.479418079 seconds


Should I submit this as an Issue on Github, or is there something else going on? I've checked both Julia 0.4.5. and 0.5 (01e3c8a). I'm on Ubuntu 16.04 64bit. 


 


Cedric St-Jean 	

Jul 19


Maybe there is some warm-up JIT time in there? If you create an Empty2 module and load it after Empty, is it also slow?
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Marius Millea 	

Jul 19


Seems it may have something to do with that. If I understood correctly what you're saying, if I create Empty2.jl defining module Empty2, I get, 

julia> addprocs();

julia> tic(); using Empty; toc()
elapsed time: 2.706353202 seconds
2.706353202

julia> tic(); using Empty; toc()
elapsed time: 0.00042397 seconds
0.00042397

julia> tic(); using Empty2; toc()
elapsed time: 0.029200919 seconds
0.029200919

julia> tic(); using Empty2; toc()
elapsed time: 0.000193097 seconds
0.000193097



That first load of Empty2 at 0.02 secs is much more in line with what loading it on a single processor takes. 
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Cedric St-Jean 	

Jul 19

Re: [julia-users] Re: Possible bug: very slow module load after addprocs()

Yes, that's what I meant. Presumably the multi-proc machinery is getting compiled at the first `using`. It's the same reason why "println(2+2)" is very slow on first use, but fast afterwards. 
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Marius Millea 	

Jul 20

Re: [julia-users] Re: Possible bug: very slow module load after addprocs()

I don't think that theory totally works, it seems to scale to some extent with the length of time to load the package itself. Another example:

julia> tic(); using PyPlot; toc()
elapsed time: 3.395904233 seconds

vs

julia> addprocs();
julia> tic(); using PyPlot; toc()
elapsed time: 13.877550518 seconds

or even:

julia> addprocs();
julia> using Empty; tic(); using PyPlot; toc()
elapsed time: 7.357315778 seconds


In any case, it can get pretty painful loading a few modules at the beginning of my parallelized scripts... 


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Cedric St-Jean 	

Jul 20

Re: [julia-users] Re: Possible bug: very slow module load after addprocs()

That does look suspicious. Maybe file an issue if there isn't one?
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Marius Millea 	

Jul 20

Re: [julia-users] Re: Possible bug: very slow module load after addprocs()

Done, see https://github.com/JuliaLang/julia/issues/17509
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