On     Mon,     30     Jun     1997    13:04:16     +1000     David     M
<[log in to unmask]> said:

>My  list  would  send  about 200-300  X  1,500-2,000  =  300,000-600,000
>messages  per day  if I  could  run it  to  full boost  (which I  cannot
>unfortunately cause  sendmail can't handle  the load and it  just queues
>and queues...)

How would multiple  SMTP workers speed up sendmail and  make it not queue
the mail due to lack of  resources? Multiple SMTP workers have three main
uses:

1. When  you deliver your mail  through a remote host  because you either
   don't have a MTA  on your LISTSERV host or it is really  not much of a
   MTA. This  is typical for  NT or older  VAXes, and for  people running
   LISTSERV at home  using their ISP's MTA.  The MTA is often  not in the
   same  location and  may have  significant  latency. This  is what  the
   feature was developed for.

2. To circumvent  various operating system limitations  or bottlenecks if
   you have a really large workload. Even within a local system, there is
   a turnaround time for socket I/O.  Even a fast disk has nonzero access
   time, etc.

3. On SMP  systems, to allow you to  use more than one CPU  to input your
   mail into the queue. Actually, even with one worker you can use 2 CPUs
   concurrently (one for the worker, one for the MTA), but this is a case
   where you  eventually need multiple  workers to use  arbitrarily large
   numbers of  CPUs. Not that anyone  actually has this problem  but this
   should be mentioned for the sake of completeness.

To  give  you   an  idea,  on  PLUM.EASE.LSOFT.COM   (3.5  million  daily
deliveries) we originally used 3 SMTP workers because of #2. Currently we
are  running  LISTSERV  on a  4-CPU  system  to  make  sure there  is  no
multiprocessing  bug in  the code  and we  increased the  number of  SMTP
workers to 5 because of #3. It would work with 2, although there would be
a small backlog if  we only had 1. We have tested it  with 30 workers and
it just increases system overhead.

  Eric