Re: very slow vectorized code

"Julian Reichl" <julesreichl@xxxxxxxxxxx> wrote in message
<fupbim$hs6$1@xxxxxxxxxxxxxxxxxx>...
I just tried commenting out all the other code within the
loop
i=1:28305
...
end
and it bought that e(:,:)=0;
statement back to 0.04s. So could this mean that there is
some memory issue?

"Julian Reichl" <julesreichl@xxxxxxxxxxx> wrote in message
<fupais$8av$1@xxxxxxxxxxxxxxxxxx>...
Hi all,
I have a problem that I can't figure out. I have two
versions of some code, one vectorised, the other not.
The
vectorised version runs WAY slower for single repeats -
~17
seconds cf. 0.3 seconds.

The profiler tells me that pretty much every line of the
code is a lot slower in the vectorised version. Even
simple
things like

e=repmat(1,1);
for i=1:28305
e(:,:)=0;
end

(the 1:28305 is a time-stepping loop and can't be
removed)
takes 0.39 s inside the vectorised code (not including
the
repmat), whereas in the command line it takes less than
a
10th of that time.
if, in the same code (the vectorised version), I
replace it
with

for i=1:28305
e=0;
end

(which is fine for this example because it's a 1x1, but
not
for my application), the time for that function drops,
again, to ~ a 10th.

I really hope someone has an idea!!!

thanks

Julian



I guess this is because JIT accelerator, i.e. your
vectorized code is quite complicated, hence the JIT
accelerator is not able to accelerate it, while your simple
loop version is in favour for the JIT to accelerate. When
you remove all other lines in the loop, the vectorized
version becomes simple as well, hence the JIT can
accelerate it now. That is why you see speed improvement.
To demonstrate this, you can try run your code with JIT on
and off by using the command:

feature accel on

or

feature accel off

to see if this is the case.

hth
Yi Cao

.

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