From:	MERC::"uunet!CRVAX.SRI.COM!RELAY-INFO-VAX" 10-APR-1993 20:44 10-APR-1993 20:44:00.00
To:	Info-VAX@KL.SRI.COM
CC:	
Subj:	Re: Interrupt Based Inter Process Communications

In article <1993Apr7.083819.371@gems.vcu.edu>, agnew@gems.vcu.edu (Brainwave Surfer) writes:
> I never use mailboxes, too much overhead.
> Event flags and global common blocks, er, sections go like gangbusters.

This "too much overhead" rumor about mailboxes is misleading.  It's true that
event flags (or hiber/wake) and global sections are generally faster; it's also
true that mailboxes are more convenient.  

On a ~6-VUP VAX (VS3100/76) a mailbox write (and read by the reader process)
takes a total of about a millisecond for a 1-K buffer.  (2k takes about 1.2 
msec)  Not terribly fast, but much faster than any real I/O device.  

Using a global section with either hiber/wake or common event flags takes about
a quarter of a second.  Definitely faster, but more work to set up.  I'll use
mailboxes whenever their attributes fit the application and where they're fast
enough -- which is most of the time.  Global sections seem to "fit"  better 
in a "broadcast" sort of application, where one process updates a section and
many readers need to look at it.  Mailboxes fit better for message passing
from a writer to a single reader.  

	--- Jamie Hanrahan, Kernel Mode Systems, San Diego CA
drivers, internals, networks, applications, and training for VMS and Windows NT
uucp 'g' protocol guru and release coordinator, VMSnet (DECUS uucp) W.G., and 
Chair, Programming and Internals Working Group, U.S. DECUS VMS Systems SIG 
Internet:  jeh@cmkrnl.com  Uucp: uunet!cmkrnl!jeh  CIS: 74140,2055