How to speed up UDP connections?

I have to different versions of Matlab (6.5 and 7.11) communicating with each other. I use UDP as protocol. But it's very slow. Just to test the whole communication thing, I created to Simulations. Sim1 adds 5 to the incoming value, Sim2 subtracts 3. So Sim1 gets an initial value (in this case 1), calculates, gives the result to Sim2, where the same thing happens and so on. I want to calculate for 100 steps and it takes about 15s! Which is too long, cause in the real simulation I have ~400000 steps. How can I optimize the whole thing? Any ideas?

remotehost = '127.0.0.1';
remoteport = 10000;
u = udp(remotehost,remoteport,'LocalPort',10001);
fopen(u);
sample_time=0.1;
A=[1 1];
for sample=1:100
     if isempty(A)
         A=fread(u,2,'double');
     end
  num_ele=length(A);    
  for i=1:num_ele/2
   udp_in(i,1)=A(2*i-1);
   udp_in(i,2)=A(2*i);
  end  

    sim('sim1',sample*sample_time)
    A=[];
    l2r=[sample*sample_time,udp_out(1)];
    fwrite(u,l2r,'double')
end
fclose(u);
delete(u);
clear u;

[Walter Roberson]

Have you tried commenting out the sim() step and seeing how long it takes? (you would probably have to initialize udp_out for that to work.)

[Stefan]

Yes, I did that, but it's the same. I also measured only the time for the fread command every step. It's ~14s. So, the UDP communciation slows the whole thing down. Any suggestions?

[Walter Roberson]

Is the fread() giving you messages about timeouts ? You do not appear to have configured for reading packet-by-packet .

[Stefan]

Yes, sometimes I get:

Warning: The specified amount of data was not returned
within the Timeout period.

How can I configure for reading packet-by-packet and what does it mean?

[Stefan]

I stil have problems with the speed. I use the following two codes:

u = udp('localhost', 5003, 'LocalPort', 5002);
u.Timeout = inf;
u.DatagramTerminateMode = 'off';
u.InputBuffersize = 4096;
u.OutputBuffersize = 4096;

fopen(u);

sample_time = 0.02;

pos = [1;7];
vel = [1;0];

tic
for sample=0:100
   pos = pos + (vel * 1);
   %fwrite(u, [pos;0], 'float');
   fwrite(u, pos, 'float');   
   vel = fread(u, [2,1], 'float');
end
toc

disp(sprintf('%0.2f: Pos[%03d,%03d], Vel[%03d,%03d]',...
   sample * sample_time,...
   pos(1,1), pos(2,1),...
   vel(1,1), vel(2,1)...
));

fclose(u);
delete(u);

clear all;

and

u = udp('localhost', 5002, 'LocalPort', 5003);
u.Timeout = inf;
u.DatagramTerminateMode = 'off';
u.InputBuffersize = 4096;
u.OutputBuffersize = 4096;

fopen(u);

sample_time = 0.02;

vel = [];

for sample=0:100
   pos = fread(u, [2,1], 'float');
   fprintf('Read: [%d %d]\n', pos);
   vel = [1;2] + [sample;sample];
   %fwrite(u, [vel;0], 'float');
   fwrite(u, vel, 'float');
end

fclose(u);
delete(u);

clear all;

But I figured out some strange things: The code written above will take around 12sec (which is too long). If you use the fwrite command with [pos;0] instead of pos the duration will be decreased to around 1sec! But the results are wrong :( Which makes sense, because Matlab will use the zero at the third position as a value for the next timestep. But this shows, that the communcation can be faster. Any ideas how to deal with that?