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Java and lsof

Update: I found that using Runtime.exec was a bad idea because if you have a 2GB VM footprint then the forked process would require 2G free memory in order to run the lsof command.  We had earlier writte a simple python http rpc server that would allow us to execute native commands(like creating a hardlink or running gunzip) from Java and I changed this code to delagate to RPC call few days back.
So the new code looks like



 public void writeTopCommandOutput(Writer writer) throws IOException {
  String rpcRes = Util.doCommandRpc(rpcUrl, "", ListUtil.create("top", "-n", "2", "-b", "-d", "0.2"));
  writer.write(rpcRes);
 }

 public void writeLsofOutput(Writer writer) throws IOException {
  String pid = getJvmProcessid();
  if (pid != null) {
   pid = pid.trim();
   String rpcRes = Util.doCommandRpc(rpcUrl, "", ListUtil.create("lsof", "-p", pid));
   writer.write(rpcRes);
  }
 }


 --------------------------------------
I was chasing a customer issue and some threads in threaddump showed that they are stuck doing filer I/O. Now I needed to chase what file paths these threads are accessing as few days ago this customer had expereinced same issue. So I asked operations to give me lsof output and as we are a distribured team and engineers dont have access to production machines.It always takes time to chase people and to a programmer this means lots of context switches and it derails your thought process.  My goal is to eliminate as many hoops in my debugging path so I wrote a JSP to get me lsof output from java. This will be a jsp accessible through internal ips only, hurray from next release onwards one more reason to avoid Operations team in chasing issues.
Here is the method I added in jsp


    public void writeLsofOutput(Writer writer) throws IOException {
        String pid = getJvmProcessid();
        if(pid!=null){
            pid = pid.trim();
            String cmdLsof = "lsof -p " + pid;
            executeCmd(writer, cmdLsof);            
        }
    }

    private void executeCmd(Writer writer, String cmd) throws IOException {
        String output;
        Process child = Runtime.getRuntime().exec(cmd, new String []{});            
        BufferedReader stdInput = new BufferedReader(new InputStreamReader(child.getInputStream()));
        while ((output = stdInput.readLine()) != null) {
            writer.write(output + "\n");
            writer.flush();
        }
        stdInput.close();
    }
    public String getJvmProcessid() throws IOException {
        String pid = null;
        File pidFile = new File(System.getenv("CATALINA_PID"));
        if(pidFile.exists()) {
            FileInputStream fin = new FileInputStream(pidFile);
            List lines = IOUtils.readLines(fin);
            fin.close();
            pid = StringUtils.join(lines.toArray());
        }
        return pid;
    } 

Comments

  1. I am surprised that your code review team is allowing you to put in such a jsp. Usually a fork/exec system command run from inside the jvm is not a good idea. Also, if you have some other problem with your java server, this lsof will not return causing more threads to be consumed in your java server waiting for this to finish.

    I think you should use some other process for generating this output.

    -Chris

    ReplyDelete
  2. Chris,

    Thanks for reviewing, I agree that in some situations like when a mount point is totally toast lsof might not return, in that case anyways there would be other tomcat threads that would be stuck in filer I/O and that tomcat would anyways need a bounce in maintenance window to come to a sane state. This JSP will be called in some rare situations and in most of the situations lsof should return and in case the first execution of jsp is stuck, I wont call another one to avoid tomcat threads being gobbled up.

    ReplyDelete

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