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From: susan@DECWRL.DEC.COM (Susan Angebranndt)
Newsgroups: comp.windows.x
Subject: Re: X V11 R1 Questions
Message-ID: <8711091935.AA04177@gilroy.dec.com>
Date: Mon, 9-Nov-87 18:00:17 EST
Article-I.D.: gilroy.8711091935.AA04177
Posted: Mon Nov  9 18:00:17 1987
Date-Received: Wed, 11-Nov-87 20:07:30 EST
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    sequent!gssc!bog@decwrl (Lee Boekelheide) writes:

    In the sample server, there is (in the 4.2bsd version) a
    complication between Dispatch, WaitForSomething and
    ReadRequestFromClient that favors servicing requests from
    clients that have complete requests in the buffers read by
    ReadRequestFromClient.

    This favoritism, induced through the flag variable
    ClientsWithInput, ignores clients that have no full requests
    in their buffers until all requests are serviced for those
    clients that have complete requests.


The flag ClientsWithInput is there so that the sample server can avoid
doing 'select()' at all costs.  Even a non-blocking select() to find
out which clients have data pending is a heavy-weight operation.  The
fairest (and simplest) algorithm we could  come up with that avoided
select calls was to process all complete requests from each select
before doing the next select call.  (We don't allow clients with
incomplete requests to be processed so that client doesn't monopolize the server.)

The results are okay with this scheme.  Running 2 plaids on my
microvax, I can still type at an xterm and get a good response.  

    Is it perhaps that the 4.2bsd ReadRequestFromClient function
    never reads in a single read() more than a packet as written
    by the client?

No, like your System V implementation, ReadRequestFromClient reads in
as much as possible.   

If your implementation of select() for System V isn't as costly as the
4.2 implementation, then you can alter the scheduler to check for
pending data more often.   

Susan