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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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