286 serial port woes

[Paul De Bra]

In article <773 at hawkmoon.MN.ORG> det at hawkmoon.MN.ORG (Derek E. Terveer) writes:
}In article <391 at w3vh.UU.NET>, rolfe at w3vh.UU.NET (Rolfe Tessem) writes:
}> [..]  I suspect that some of your performance
}> problems stem from the fact that 1 meg is the absolute
}> minimum the system will run with -- as you start adding
}> memory, performance increases dramatically.
}
}I agree.  I was running (at work, believe it or not!) a xenix system with 512K
}of memory, and boy was that slow!!!!  Combine the small, no; make that
}"miniscule", amount of memory with the file advisory/mandatory locking problems
}in that release of xenix (2.1.3, ugh!), and i had processes that would take in
}excess of a week to run instead of 5 minutes.  
}
}I would almost venture to say, in an off the cuff manner, that you get
}exponential performance improvements in all of the various flavours of unix as
}you progress from the basement (0.5M) to something like 2 or 3Meg.  The
}performance improvements are still there beyond that, they just aren't as
}noticable.  (Of course, i don't have the money to spend on memory, so i can't
}*really* say how noticeable these improvements are beyond a "workable" system,
}i.e., one that has approximately 2.5-3.5M of memory.  Perhaps someone else can
}chat on the 4M+ range.. (:-))
}

Something is missing in the above reasoning. Adding memory does not give
your cpu more MIPS!

A Xenix or Unix box with very little memory suffers in 2 ways:
1) there is not enough memory to have a reasonably sized I/O buffer pool,
   so most of the file I/O involves real disk I/O.
2) there is not enough memory to hold all processes so there is a lot of
   swapping.

By adding memory these 2 problems go away. With System V/AT the buffer pool
can grow very large, so file I/O keeps improving, mostly because disk I/O
is very slow in V/AT, so you better have a large cache. With Xenix the
buffer pool tops at about 1Mbyte (because of system-tables being limited
to 64K). But above 512K the additional gain is really minimal, since disk I/O
is fairly fast. Also, adding memory means that once you reach some amount
of memory all your processes will fit in memory, so there will be no more
swapping.

Once you reached a state where you have a large buffer pool and no swapping
any additional memory you add is a waste and will not contribute to
performance in any way in a Xenix system (it may still do something in V/AT
due to problems with the brk and sbrk implementation).

Having a need for about 3 Mbytes on my system, and having bought 5Mbytes
+ 640K when memory was *very* cheap, I use 2Mbytes as a ram-disk, which was
the only way to further improve performance.

Paul.
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