Re: Memory v's CPU

From: Anthony Baxter <>
Date: Wed, 28 May 1997 14:36:13 +1000

>>> Walter Klomp wrote
> What is typically a big cache. Would a for instance a Pentium 60 with =
> 512k and Raid-controller with 8Mb be sufficient for a lets say 6Gb Cache =
> over a 256k LL and max 150 clients attached? What r the guidelines =
> here ?

I don't know that anyone has collected numbers of what sort of things are
acceptable - it really does depend on a wide number of variables. As well
as the obvious ones like number of clients and amount of disk, you have to
consider things like

the OS - how efficient is the OS? How good is it's networking?
the hardware - what I/O speed can you get? If it's slower to get stuff
                     off disk, this slows everything down, and means more
                     connections up for longer.
the external links - if they're busy, transfers take longer, which means the
                     connections hang around longer, which means more memory.
the client links - again, slower links means that the connections stay
                     around longer. What mix of ICP vs. HTTP will you have?
                     Make sure you have enough bandwidth to the cache (is
                     ethernet enough?)
the user profile - what sort of stuff will they be doing? Downloading
                     software? Viewing images? Accessing corporate information
                     (eg, vendor web pages)?

This really ends up as a complex question, along the lines of "how do I
build a fast timesharing system?" An obvious, although not all that
helpful, suggestion is to put reasonable system up, and just keep plugging
in memory until you find it's fast enough (watch the paging numbers from
vmstat). You can use the other normal system monitoring tools to watch
performance as well - iostat, top, sar (if your flavour of Unix has
it), and the like.

More random points -
When you're doing this, get hardware that can expand - maybe that can
support multiple CPUs (or upgradable ones) (although more than 2 CPUs
is probably pointless). Don't fill all the memory slots with 4Mb memory
modules - spend a bit more and leave space for more. Maybe go for
fastwide SCSI (more spindles are good) - if you're using a storage
solution like RAID, don't use RAID 5 or mirroring, that slows down
writes. Striping is good. Prestoserve is good.

Oh, and if you're wanting to build a real cache, don't put other stuff on
the box as well.

Received on Tue May 27 1997 - 21:37:40 MDT

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