Memory useage variations, PIII vs P4, or what?

Hi,

I have been monitoring memory usage on two PCs to gather info. re. a RAM
upgrade, and found discrepancies in the amount of memory a module takes
on two PCs.

1) Dell Dimension 8300, P4 3GHz, 1GB RAM, XP Pro
2) Dell Latitude CPxJ, PIII 750MHz, 512MB RAM, XP Pro

If I run task manager and look at the processes tab, on each machine,
they show that similar processes are using different amounts of memory.

Examples
avgcc.exe uses 10,860K on the Dimension, and 9,500K on the Latitude
zlclient.exe 3,196 2,820
services.exe 3,136 2,936
taskmgr.exe 4,396 3,548

avgcc.exe is part of the AVG anti-virus package and I'd assume that it
would use the same amount of memory on any machine. That assumption is
wrong, obviously, but why should that be?
(I'm only giving AVG as a example, every process uses more memory on the
Dimension.)

Any thoughts?

Regards, John

 

Dimension has more RAM therefore programs use more memory. If you ran this
same test on an identical machine with 64MB RAM what do you think the reults
would be

 

I don't see that. Why should programs use more memory just because it's
there?

 

Demand-based paging. Think of RAM as simply a disk cache. A program is
loaded from disk into memory. But the whole program is rarely ever
loaded into memory these days (like they used to be in the old DOS
days). Only the portion of the program that is required now is loaded in
now. If other parts of the program are required later, then those parts
are paged in later, while unused portions of that program are discarded
back to disk. The more ram you have, the less that has to be discarded
right away back to disk.

Yousuf Khan

 

Ok... If you are talking about virtual memory, then I still don't see
it. These machines have quite a lot of memory and no big application is
running during my observations, so they are not anywhere near 'full'.
As far as I can see no paging is happening.

Surely the same process would have the same size working set on both
machines? There is memory to spare and no paging is happening, unless I
am missing something.

 

There could be a lot of reasons. One reason is that a lot of software
auto-tunes its memory usage based upon memory availability. If more memory
is available, the programs make size/speed tradeoffs in favor of speed
rather than size.

Another is memory pressure. The operating system is constantly trying to
use your RAM for the most ideal purpose. Memory pressure on the machine with
less RAM may simply have caused more of the initialization code or unused
assemblies attached to used assemblies to be discarded than on the machine
with more memory.

http://getdotnetco.web101.discountasp.net/GdncStore/free/Articles/The Memory Mystery.htm

DS

 

Thanks David. I understand what you say about memory tuning, although I
am a little surprised that every process uses more memory on the bigger
machine. But maybe it's very common practice to do this tuning?
Re. memory pressure, I suppose that the OS might leave more of each
process in memory if there is more 'spare' memory. I'm getting into
speculation now so I'd better stop and do a bit more reading.
That site that looks interesting. I'll read it thoroughly when I get a
minute. Inevitably, this subject is a little more complex than I perhaps
expected. ;-)

 

Yes, it is. You're making some very simple assumptions about some very
complex code. For example, every program, more or less, uses the memory
allocation routines provided by the system library. They may be tuned in
this way.

Correct. That's exactly what it does.

Yes. The code, both in the applications and in the OSes memory manager,
are so complex that you can't make simple assumptions.

DS

 

Ah, yes. Good point.

Yes, as an IT person (although I haven't done programming for a good few
years) I should have been more careful of doing that. ;-)

I still find it a little mind-boggling at times, to think that my
current PC has over 1500 times as much memory as my first (XT) PC!

John

 

Sure as far as you're concerned no paging is happening, but in fact
there's paging operations happening thousands of times per second on
these machines. Most of them can be satisfied without going to disk, due
to various optimizations.

One such paging optimization is extremely simple, almost bordering on
stupidly simple. It involves the fact that executable files will never
change on the disk. If you think about it, executables are operationally
read-only files on disk, you only read them into memory and that's it;
they will never need to be written back to disk because they never
change. Well, they may change if you upgrade them to a newer version or
apply patches and that sort of thing, or if perhaps a virus infects it
-- but those are unusual operations. So any stale page that is mapped to
an executable file on disk, will simply get discarded, it doesn't need
to be written back to disk; when that page is needed in memory again,
you just read it in from the original executable file again.

Another optimization that can be performed involves paging back in from
memory. The OS continuously scans the pages in memory for activity, and
it marks any pages not used recently as "free". Once the area is marked
as free, it's now available to be overwritten from somewhere else. But
often times, it seems pages that have been marked as free and have not
yet been overwritten by another page, can simply be brought back into
the active pool without reading back from disk.

The OS sets paging limits based how much physical memory is on the
machine. For example, it might not so aggressively mark a page as free
if there is more physical memory available, so you'll end up seeing more
old pages being kept resident. Also another thing it might do more
reading ahead if there is more memory available, so loading in more
pages ahead of time, even if they aren't quite needed yet. Since going
to disk is a high-latency operation, the OS will tend to read as many
pages ahead as possible -- thus minimizing the number of disk
operations. It's better to read ten pages in one operation than it is to
read ten pages individually.

Yousuf Khan

 

Yeah. I compare the PC I have now to the 'PC' I had in 1980. 15,000
times the memory, 300,000 times the mass storage space, and 9,000 times the
processing power. I wonder what I'll have in 2030.

DS

 

I think perhaps I'd use slightly different terminology, but I see what
you are getting at. A lot of what you describe I'd call 'memory
management', or something like that. Paging, as I understand it, means
writing memory blocks to the paging file on disk, and reading them back
in again when required.
Thanks for the advice anyway. This is a complex subject and I think a
bit more reading is required.

John

 

To me, paging and memory management are the same thing. The specific act
of paging memory to disk is just a subset of all of the operations that
can really be done with paging.

Yousuf Khan