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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 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.

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).

----- (Gregg Wonderly)

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