CODEWITHSUNDEEP
c++linuxperformanceshell
Wallace
Wallace

Reputation: 651

Performance Issue in Executing Shell Commands

In my application, I need to execute large amount of shell commands via c++ code. I found the program takes more than 30 seconds to execute 6000 commands, this is so unacceptable! Is there any other better way to execute shell commands (using c/c++ code)?

    //Below functions is used to set rules for 
    //Linux tool --TC, and in runtime there will 
    //be more than 6000 rules to be set from shell
    //those TC commans are like below example:

    //tc qdisc del dev eth0 root 
    //tc qdisc add dev eth0 root handle 1:0 cbq bandwidth 
    //   10Mbit avpkt 1000 cell 8
    //tc class add dev eth0 parent 1:0 classid 1:1 cbq bandwidth
    //   100Mbit rate 8000kbit weight 800kbit prio 5 allot 1514 
    //   cell 8 maxburst 20 avpkt 1000 bounded
    //tc class add dev eth0 parent 1:0 classid 1:2 cbq bandwidth 
    //   100Mbit rate 800kbit weight 80kbit prio 5 allot 1514 cell 
    //   8 maxburst 20 avpkt 1000 bounded
    //tc class add dev eth0 parent 1:0 classid 1:3 cbq bandwidth 
    //   100Mbit rate 800kbit weight 80kbit prio 5 allot 1514 cell 
    //   8 maxburst 20 avpkt 1000 bounded
    //tc class add dev eth0 parent 1:1 classid 1:1001 cbq bandwidth 
    //   100Mbit rate 8000kbit weight 800kbit prio 8 allot 1514 cell 
    //   8 maxburst 20 avpkt 1000
    //......

    void CTCProxy::ApplyTCCommands(){
        FILE* OutputStream = NULL;        

        //mTCCommands is a vector<string>
        //every string in it is a TC rule               
        int CmdCount = mTCCommands.size();
        for (int i = 0; i < CmdCount; i++){            
            OutputStream = popen(mTCCommands[i].c_str(), "r");
            if (OutputStream){
                pclose(OutputStream);
            } else {
                printf("popen error!\n");
            }     
        }
    }

UPDATE
I tried to put all the shell commands into a shell script and let the test app call this script file using system("xxx.sh"). This time it takes 24 seconds to execute all 6000 entries of shell commands, less than what we toke before. But this is still much bigger than what we expected! Is there any other way that can decrease the execution time to less than 10 seconds?

Upvotes: 0

Views: 903

Answers (2)

Mats Petersson
Mats Petersson

Reputation: 129514

So, most likely (based on my experience in a similar type of thing), the majority of the time is spent starting a new process running a shell, the execution of the actual command in the shell is very short. (And 6000 in 30 seconds doesn't sound too terrible, actually).

There are a variety of ways you could do this. I'd be tempted to try to combine it all into one shell script, rather than running individual lines. This would involve writing all the 'tc' strings to a file, and then passing that to popen().

Another thought is if you can actually combine several strings together into one execute, perhaps?

If the commands are complete and directly executable (that is, no shell is needed to execute the program), you could also do your own fork and exec. This would save creating a shell process, which then creates the actual process.

Also, you may consider running a small number of processes in parallel, which on any modern machine will likely speed things up by the number of processor cores you have.

Upvotes: 2

kworr
kworr

Reputation: 3674

You can start shell (/bin/sh) and pipe all commands there parsing the output. Or you can create a Makefile as this would give you more control on how the commands whould be executed, parallel execution and error handling.

Upvotes: 1

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