convert for loop into parfor loop
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Can somebody point out how I have to modify the code below to allow it being used in a parfor loop
H = height(RTImport)
location = 'location..'
ds = datastore(location)
TT = tall(ds)
func = @(x) mean(x,'omitnan');
x1{H} = {};
x2{H} = {};
tic
for i = 1:H
Strt = minutes(RTImport.Start(i))
endT = Strt + minutes(1)
S = timerange(Strt,endT,'closed')
TT1 = TT(S,:)
Strt = Strt + minutes(1)
endT = Strt + minutes(1)
S = timerange(Strt,endT,'closed')
TT2 = TT(S,:)
x1{i} = varfun(func,TT1,'OutputFormat','table')
x2{i} = varfun(func,TT2,'OutputFormat','table')
gather(x1{:},x2{:})
end
toc
I get the information unable to classify x1 in the body of the parfor. Going through the information, I couldnt really figure out what wrong with it...Appreciate any help
6 个评论
Rik
2020-11-17
Did you try pre-allocating it to the correct size? And why aren't you storing the result of gather in a variable? And why are you only using 3 semicolons? There is no reason to omit them if you are already at the stage that you worry enough about the performance that you want to use parfor.
Lutetium
2020-11-17
Rik
2020-11-17
I prefer a more explicit pre-allocation like this:
x1=cell(1,H);
Your call to gather doesn't actually do anything here. It has an output, but that isn't stored, so your x1 variable remains unchanged. It is very rare in Matlab that a call to a function will modify the input in the calling workspace. I don't know of any examples outside the symbolic toolbox with functions like assume. In short: your call doesn't do anything, so it can be omitted. If you need it, you should probably use this instead:
[x1{i},x2{1}]=gather(x1{i},x2{i});
(but I am not sure why you would need it)
A small note: because you're using a parallel loop you can't be sure about the order, and no iteration can depend on any other (so the concept of needing to gather variables before doing the next iteration may be flawed). For some calculations it takes more time to copy the variables to all the separate workers that to do the calculation in a single thread. This overhead is the reason why parfor is in some cases slower than for.
Lutetium
2020-11-17
Lutetium
2020-11-17
Rik
2020-11-17
You're welcome.
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