Performance loss using Matlab parfeval? - matlab

Question

I have a relatively big file and I would like to make an interactive plot using GUIDE which plots a segment of the file and upon scrollEvent the window gets updated. (the data is appropriately replotted).
To this end a buffer of size 4 times that of the window is fetched, and when the window's center reaches 75% of the buffer, the buffer is refetched so that the window is at the center of the new buffer.
The problem with this is that when using fread it of course blocks until done.(which is visually disturbing)
What I tried is to create a separate readData function which gets called with f = parfeval(gcp(),#readData,1,datafile) and on [~,buffer]=fetchNext(f) the buffer is persisted on handles.datafile.
Problem: Even though in my example (see below) readData only gets called once, every subsequent plotting is incredibly slow (10x the runtime of when not using parfeval).
Example:
./test.dat was generated by dd if=/dev/urandom of=test.dat bs=100000 count=1024
**This is the synchronous code to get the asynchronous one, comment out the parfeval and fetchNext function definitions shadowing the parallel ones.
function test()
f = figure('Toolbar','none','Menubar','none');
ax = axes(f);
data = struct( 'file','./test.dat',...
'buffer',[],...
'window',[0,100000]);
p = gcp();
data.fileReader = parfeval(p,#readData,1,data);
handles = struct('axes',ax,'figure',f,'data',data);
guidata(f,handles);
set(f,'WindowScrollWheelFcn',#scrollHandler);
end
function f = parfeval(~,fun,~,in1)
f = struct('output',fun(in1));
end
function [id,out] = fetchNext(f,varargin)
id = 1;
out = f.output;
end
function buffer = readData(data)
file = fopen(data.file,'r');
buffer = fread(file,[128,400000],'int16');
fclose(file);
end
function scrollHandler(hObject, eventdata, ~)
handles = guidata(hObject);
ax = handles.axes;
C = get (ax, 'CurrentPoint');
XLim = get(ax, 'XLim');
YLim = get(ax, 'YLim');
if XLim(1)<=C(1,1) && XLim(2)>=C(1,1) && ...
YLim(1)<=C(1,2) && YLim(2)>=C(1,2)
tic;
window = handles.data.window;
if isstruct(handles.data.fileReader) || handles.data.fileReader ~= -1
fprintf('Reading from file\n');
[~,handles.data.buffer] = fetchNext(handles.data.fileReader);
handles.data.fileReader = -1;
end
if eventdata.VerticalScrollCount > 0 && window(2) < 399000
window = window + 1000;
else
if window(1) > 1000
window = window - 1000;
end
end
handles.data.window = window;
guidata(hObject, handles);
plot(ax,handles.data.buffer(65,window(1)+1:window(2)));
toc
end
end

Answer 1

This is indeed very mysterious. The problem seems to be some sort of interaction with guidata and the parallel.FevalFuture returned by parfeval. As a workaround, you can simply avoid overwriting that element of the struct. In other words, remove the line
handles.data.fileReader = -1;
You'll also need to check handles.data.fileReader.Read to make sure you don't attempt to call fetchNext again on an already-completed future.