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Interprocess communication

The Question is:

External constaints have forced our processing to be divided
between two processes. The relationship is strictly
a client - server one, process A wants a service from
process B and can do no useful work until it completes. Is
there any method of interprocess communication (mailboxes,
event flags, locks, etc) which will minimize the cost of the
context switch between the two processes (assuming that any
actual data is transferred via shared memory).

Thanks in advance.

The Answer is:

  Event flags and locks are the "lightest weight" communications mechanisms.
  (Hardware bitlocks are the lightest, but they do not include a notification
  mechanism.)

  The way to minimize context switching is to divide the work appropriately
  between the processes -- an application that "ping-pongs" small messages
  will run more slowly than an application design that "batches" requests in
  larger units.