Performance of system/dos function

Question

Xiangrui Li 2017-5-3

4
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/338553-performance-of-system-dos-function

评论： Horshack Horshack 2021-4-20

在 MATLAB Online 中打开

[status, cmdout] = system(cmd)

has been reported to be slow for a while (see this and this), but there seems no satisfying solution yet.

Here is what I tested for Windows command reg.exe, trying to figure out a workaround.

 cmd = 'reg.exe query HKEY_LOCAL_MACHINE\HARDWARE /s'; % as an example
 tic; [status, cmdout] = system(cmd); toc
  Elapsed time is 3.517736 seconds.
 whos cmdout
  Name        Size            Bytes  Class    Attributes
  cmdout      1x95249        190498  char 
 tic; [status, cmdout] = dos(cmd); toc
  Elapsed time is 3.512847 seconds.
 tic; cmdout = evalc(['! ' cmd]); toc
  Elapsed time is 3.509961 seconds.

As we can see, system(), dos() and evalc('!') have similar performance. When I test the same cmd in other programs (Octave and python), it takes about 0.6 to 0.7 seconds. Running it at Windows command prompt takes less than a second based on my eye test. Then I tried to redirect output to a file:

 tic; [status, cmdout] = system([cmd ' >myFile']); toc
  Elapsed time is 0.886915 seconds.

This improves the performance significantly. The cmdout is empty now as expected.

Does this suggest there should be a way to improve system() for dealing with cmdout?

Is there a way to redirect cmdout to a Matlab string, so we can avoid to read and delete the temp file?

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Xiangrui Li 2017-5-9

在 MATLAB Online 中打开

I found that Windows's findstr helps at this case.

ftdiKey = 'HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\FTDIBUS';
tic, [~, str] = dos(['reg.exe query ' ftdiKey ' /s']); toc
 Elapsed time is 1.442744 seconds.
ports = regexp(str, 'COM\d{1,3}(?=\s)', 'match'); % get all FDTI VCP
disp(numel(str))
  17740

Following is much fast, but does not work for early reg.exe:

tic, [~, str] = dos(['reg.exe query ' ftdiKey ' /s /f PortName']); toc
Elapsed time is 0.057759 seconds.
disp(numel(str))
  1631

findstr is similar to unix grep, although may not as powerful as grep:

tic, [~, str] = dos(['reg.exe query ' ftdiKey ' /s | findstr PortName']); toc
 Elapsed time is 0.141830 seconds. 
disp(numel(str))
   345

The file approach is similar to findstr:

tic, [~, str] = dos(['reg.exe query ' ftdiKey ' /s >myFile']);
fid = fopen('./myFile'); str = fread(fid, '*char')'; fclose(fid); toc
 Elapsed time is 0.130066 seconds.

Anyway, it seems the system/dos function has a huge room to improve. My wild guess is that the pre-allocation for the output may not be dealt with efficiently.

Xiangrui Li 2018-5-17

The jsystem at Github has fixed the hanging problem.

Horshack Horshack 2021-4-20

在 MATLAB Online 中打开

I'm having the same issue. To demonstrate, I wrote a simple C program 'printstdout' that prints the number of characters it's been told to via a command line argument:

#include <stdio.h>
#include <stdlib.h>
int main(int argc, char* argv[]) {
    int     numCharsToPrint = argc >= 2 ? atoi(argv[1]) : 0;
    int     numCharsPrintedThisLine;
    numCharsPrintedThisLine = 0;
    while (numCharsToPrint-- > 0) {
        printf("*");
        if (++numCharsPrintedThisLine == 78) {
            numCharsPrintedThisLine = 0;
            if (numCharsToPrint-- > 0)
                printf("\n");
        }
    }
    return 0;
}

And tested it with the following script:

    numChars = uint32(1);
    while (numChars <= 2^20)
        fprintf("numChars = %7d: ", numChars);
        tic; [exitCode, output] = system(['printstdout ' num2str(numChars)]); toc;
        numChars = numChars*2;
    end

Here's the output for Matlab:

numChars =       1: Elapsed time is 0.035415 seconds.
numChars =       2: Elapsed time is 0.034140 seconds.
numChars =       4: Elapsed time is 0.033591 seconds.
numChars =       8: Elapsed time is 0.034396 seconds.
numChars =      16: Elapsed time is 0.033863 seconds.
numChars =      32: Elapsed time is 0.033824 seconds.
numChars =      64: Elapsed time is 0.033170 seconds.
numChars =     128: Elapsed time is 0.033617 seconds.
numChars =     256: Elapsed time is 0.033296 seconds.
numChars =     512: Elapsed time is 0.034161 seconds.
numChars =    1024: Elapsed time is 0.032989 seconds.
numChars =    2048: Elapsed time is 0.033904 seconds.
numChars =    4096: Elapsed time is 0.602469 seconds.
numChars =    8192: Elapsed time is 0.703521 seconds.
numChars =   16384: Elapsed time is 0.903753 seconds.
numChars =   32768: Elapsed time is 1.304120 seconds.
numChars =   65536: Elapsed time is 2.105577 seconds.
numChars =  131072: Elapsed time is 3.708676 seconds.
numChars =  262144: Elapsed time is 6.916353 seconds.
numChars =  524288: Elapsed time is 13.246939 seconds.
numChars = 1048576: Elapsed time is 26.452178 seconds.

And here is the output from Octave:

numChars =       1: Elapsed time is 0.0197449 seconds.
numChars =       2: Elapsed time is 0.0113029 seconds.
numChars =       4: Elapsed time is 0.0113032 seconds.
numChars =       8: Elapsed time is 0.0113418 seconds.
numChars =      16: Elapsed time is 0.011956 seconds.
numChars =      32: Elapsed time is 0.010741 seconds.
numChars =      64: Elapsed time is 0.011215 seconds.
numChars =     128: Elapsed time is 0.0108809 seconds.
numChars =     256: Elapsed time is 0.011487 seconds.
numChars =     512: Elapsed time is 0.010798 seconds.
numChars =    1024: Elapsed time is 0.012202 seconds.
numChars =    2048: Elapsed time is 0.0105009 seconds.
numChars =    4096: Elapsed time is 0.0121739 seconds.
numChars =    8192: Elapsed time is 0.01156 seconds.
numChars =   16384: Elapsed time is 0.0121598 seconds.
numChars =   32768: Elapsed time is 0.0127161 seconds.
numChars =   65536: Elapsed time is 0.014755 seconds.
numChars =  131072: Elapsed time is 0.0170429 seconds.
numChars =  262144: Elapsed time is 0.023046 seconds.
numChars =  524288: Elapsed time is 0.0346019 seconds.
numChars = 1048576: Elapsed time is 0.0581162 seconds.

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Performance of system/dos function

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