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SubjectRe: [ Matthias Sattler ] Bad 1.3.95 interactive performance (fwd)

> In your message dated: Mon, 29 Apr 1996 20:39:41 +0200
> >In addition to that, I'm a bit alarmed because no one else seems to see this
> >problem :-(
> Well, looking at the output of free, it looks to me like you don't have
> enough physical memory for you set up. And, considering you seem to have
> a single disk, I am amazed (in a sense) that the disk head is not protruding
> from the side of your machine by now!

I use this setup (with this virtual memory usage) since 7 or 8 months
without any performance problems. I can even start a kernelcompile and still
work nicely. It seems that much of the virtual memory allocated is never
used. In addition to that everything works fine after a reboot, the problems
appear after some inactive!! (just the usual cronjobs) hours.

> I am sure that most people on this list understand the concepts of swapping
> and paging. What some people might not understand are the issues with the
> use of VM, the number of partitions your disk(s) has/have and the disks
> that have swap space on them verses those that have swap and active text
> on them.

I've put swap in the middle of the disk to minimize head movement.

> In recent times, it was decided that there was no reason to swap text
> (remember the sticky bit) because it was faster to just tell the VM manager
> that the memory for text was free and read it back from the original disk
> location later (text file busy scrolling up the screen...)
> The problem is that by doing this, the disk I/O for an active system that
> is using VM to manage the address space(s) must now traverse a larger physical
> surface area to get the text for all the active programs and then go back to
> the swap partition to read/write data, than it did when it was all in one
> partition! So, when you have a single disk and swapping starts, you can bet
> that it could take quite some time to accomplish the VM shuffle required to
> handle all of the address spaces that are active.
> My machine has 2 1GB scsi disks (CONNORs), and 1 2GB IDE disk (MAXTOR).
> The day that I moved all of my swap to the IDE, I decided I did not need
> more phsical memory. It used to take 10-15secs to switch virtual desktops
> in fvwm. Now it takes 2-3secs! And that it swaping from a file that is
> located on a fat file system.
> It might be interesting to turn off the special case for text some day
> and see what happens to swapping when the head can hover over a single
> partition again!
> Ever heard of a paging drum? If you know someone who has one of those really
> small SCSI or IDE drives (40-200 megs), you might think about picking one up
> and using it for your swap partition. It will at least keep the head from
> having to consistantly wander around between text and data sources during
> swapping. And maybe, like it did for me, you'll get a sizeable performance
> improvement without having to buy more physical RAM (which I think is the
> source of your wild swapping problem).

I've though about buying a disk to swap on, but I never really did it
because system performance was ok for me.

> -----
> (Gregg Wonderly)

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