Re: Memory trashing? Virtual memory question.

albert@spenarnc.xs4all.nl wrote:

In article <nnd$0b021237$626a8881@55133cc8c31c4c51>,
 <albert@spenarnc.xs4all.nl> wrote:

I remember running a program intended for a pdp 11 for a vax.

<SNIP>
 
I confused the issue with talking too much.
 
I run on a AMD FX870 with 16 Gbyte RAM

Have you checked how much RAM is available?  Things like integrated
graphics or hole in address space for peripherials can eat hundreds
of megabytes.  Linux takes part of memory for itself.  Use 'free'
to know how much memory is in use.  'top' can tell you how much
memory is in use for various processes.

I run as root, and killed the gimp prime search and firefox browser.
I also give absolute priority to Forth (nice negative).
 
root@cherry:/home/albert/PROJECT/ciforths/ciforth# nice -20 lina16G++
 
AMDX86 ciforth beta 2022Feb17
WANT ELAPSED
ASSEMBLERi86-HIGH : (WARNING) NOT PRESENT, THOUGH WANTED
 OK
MARK-TIME HERE 16,000,000,000 ERASE ELAPSED .mS
 12810.175mS  OK
MARK-TIME HERE 16,000,000,000 ERASE ELAPSED .mS
 394536.356mS  OK
BYE
 
The second time it takes 30 times longer!

You probably exceeded available memory.  On first write Linux
allocates you a fresh zeroed page.  To get memory Linux may have
to swap something else, but that happens only when memory use
exceeds available RAM.  But on second run Linux have to bring
back what if swapped.  Actually, your second time looks quite
good: linear walk trough array bigger than RAM is a classic
example of of worst case for swapping.  Classic algorithm
is expected to swap the whole 16GB, I do not think that your
hardware is capable of doing it in 39 seconds.

Another possibility is delayed write: Linux may anticipate need
for more memory and schedule some parts to be written to disc.
But this normally does not happen immediately, but with some
delay (used to be 30 seconds).  You can test this by waiting
few minutes between the two commands.

Theories about working set doesn't cut it.
Also the system freezes, I expect better from a system with 8 cores,
but maybe this is caused by nice.

You can not expect much when system runs out of memory.  Almost
anything may need memory allocation and freezes waiting for
free memory (that is for other things to be swapped out).
Nice makes things worse, as memory eater is scheduled first
and eats available RAM and swap bandwidth.

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                              Waldek Hebisch