Chapter 7. Identifying application read socket buffer bottlenecks

Procedure

1. Use the nstat utility to query the TcpExtTCPRcvCollapsed and TcpExtRcvPruned counters:

   # nstat -az TcpExtTCPRcvCollapsed TcpExtRcvPruned
   #kernel
   TcpExtRcvPruned            0         0.0
   TcpExtTCPRcvCollapsed      612859    0.0

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2. Wait some time and re-run the nstat command:

   # nstat -az TcpExtTCPRcvCollapsed TcpExtRcvPruned
   #kernel
   TcpExtRcvPruned            0         0.0
   TcpExtTCPRcvCollapsed      620358    0.0

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3. If the values of the counters have increased compared to the first run, tuning is required:

   • If the application uses the setsockopt(SO_RCVBUF) call, consider removing it. With this call, the application only uses the receive buffer size specified in the call and turns off the socket’s ability to auto-tune its size.
   • If the application does not use the setsockopt(SO_RCVBUF) call, tune the default and maximum values of the TCP read socket buffer.

4. Display the receive backlog queue (Recv-Q):

   # ss -nti
   State   Recv-Q   Send-Q   Local Address:Port   Peer Address:Port   Process
   ESTAB   0        0        192.0.2.1:443        192.0.2.125:41574
         :7,7 ... lastrcv:543 ...
   ESTAB   78       0        192.0.2.1:443        192.0.2.56:42612
         :7,7 ... lastrcv:658 ...
   ESTAB   88       0        192.0.2.1:443        192.0.2.97:40313
         :7,7 ... lastrcv:5764 ...
   ...

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5. Run the ss -nt command multiple times with a few seconds waiting time between each run.

   If the output lists only one case of a high value in the Recv-Q column, the application was between two receive operations. However, if the values in Recv-Q stays constant while lastrcv continually grows, or Recv-Q continually increases over time, one of the following problems can be the cause:

   • The application does not check its socket buffers often enough. Contact the application vendor for details about how you can solve this problem.

   • The application does not get enough CPU time. To further debug this problem:

     1. Display on which CPU cores the application runs:

        # ps -eo pid,tid,psr,pcpu,stat,wchan:20,comm
            PID     TID PSR %CPU STAT WCHAN                COMMAND
        ...
          44594   44594   5  0.0 Ss   do_select            httpd
          44595   44595   3  0.0 S    skb_wait_for_more_pa httpd
          44596   44596   5  0.0 Sl   pipe_read            httpd
          44597   44597   5  0.0 Sl   pipe_read            httpd
          44602   44602   5  0.0 Sl   pipe_read            httpd
        ...

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        The PSR column displays the CPU cores the process is currently assigned to.

     2. Identify other processes running on the same cores and consider assigning them to other cores.