What does sbwait mean in top ?

[Robert Watson]

On Sun, 6 Feb 2005, Jon Drews wrote:

>  What does sbwait mean in top? Is it related to a sysctl setting? I have
> looked in the man page for top, at William LeFebvres site (
> http://www.groupsys.com/top ), in Evi Nemeth's "UNIX System
> Administration Handbook" and Googled in general.  Does anyone know of a
> URL that gives an explanation of sbwait and for that matter semwait too. 

The sbwait wchan is present when a thread has invoked the in-kernel
sbwait() function to wait for a socket event.  It's used in a number of
situations, but the main ones are:

- The thread is trying to send on a blocking socket, but there's
  insufficient socket buffer space, so it must wait for space.  This might
  occur if it has managead to max out the bandwidth available to a TCP
  connection, or flow control is in use and the receiver does not wish to
  receive more data yet.

- The thread is trying to receive on a blocking socket, but there's not
  enough data to satisfy the read request, so it must wait for data to be
  received.  It might be waiting for a remote TCP sender to have data
  available, or for in-flight data to arrive.

Robert N M Watson