Free memory level in 2.6.16?

Free memory level in 2.6.16?

[Steve Bergman]

Due to some problems I was having with the CentOS4.4 kernel, I just
moved a box (x86 with 4GB ram) to 2.6.16.29 from kernel.org.

All is well, but I am curious about one thing.  This is a fairly memory
hungry box, serving about 40 gnome desktops via xdmcp.  All VM settings
are at the default.  Swappiness=60, min_free_kbytes=3831.

However, it seems to seek out about 150MB for the level of free memory
that it maintains.  Typically I see somewhere between 100MB an 500MB in
swap, buffers+cache is about 500MB, and 150MB is free.

If I cat from /dev/md0 to /dev/null, the free memory does go down, to
25MB or so,  but then I can watch as it seeks out about 150MB of free
memory.

To me, free memory is wasted memory.  Is this a bug or a feature?

Thanks for any enlightenment.

Re: Free memory level in 2.6.16?

[Andi Kleen]

Steve Bergman <sbergman@rueb.com> writes:

> Due to some problems I was having with the CentOS4.4 kernel, I just
> moved a box (x86 with 4GB ram) to 2.6.16.29 from kernel.org.
> 
> All is well, but I am curious about one thing.  This is a fairly memory
> hungry box, serving about 40 gnome desktops via xdmcp.  All VM settings
> are at the default.  Swappiness=60, min_free_kbytes=3831.
> 
> However, it seems to seek out about 150MB for the level of free memory
> that it maintains.  Typically I see somewhere between 100MB an 500MB in
> swap, buffers+cache is about 500MB, and 150MB is free.
> 
> If I cat from /dev/md0 to /dev/null, the free memory does go down, to
> 25MB or so,  but then I can watch as it seeks out about 150MB of free
> memory.
> 
> To me, free memory is wasted memory.  Is this a bug or a feature?

Normally it keeps some memory free for interrupt handlers which
cannot free other memory. But 150MB is indeed a lot, especially
it's only in the ~900MB lowmem zone.

You could play with /proc/sys/vm/lowmem_reserve_ratio but must
likely some defaults need tweaking.

-Andi

Re: Free memory level in 2.6.16?

[Steve Bergman]

On Thu, 2006-10-05 at 22:01 +0200, Andi Kleen wrote:

> 
> Normally it keeps some memory free for interrupt handlers which
> cannot free other memory. But 150MB is indeed a lot, especially
> it's only in the ~900MB lowmem zone.
> 
> You could play with /proc/sys/vm/lowmem_reserve_ratio but must
> likely some defaults need tweaking.

Thank you for the reply, Andi.  This kernel is compiled with the .config
from the original FC5 release, which used kernel 2.6.15.  I just ran
"make oldconfig" on it and accepted the defaults.

So it is, I believe, a 4GB/4GB split.  Does that make a difference?

Thanks,
Steve Bergman

Re: Free memory level in 2.6.16?

[Andi Kleen]

> Thank you for the reply, Andi.  This kernel is compiled with the .config
> from the original FC5 release, which used kernel 2.6.15.  I just ran
> "make oldconfig" on it and accepted the defaults.

I meant in the source. There are no tunables for this in
.config
 
> So it is, I believe, a 4GB/4GB split.  Does that make a difference?

The kernel.org kernel doesn't support 4/4 split.

-Andi

Re: Free memory level in 2.6.16?

[Steve Bergman]

On Thu, 2006-10-05 at 22:01 +0200, Andi Kleen wrote:

> 
> You could play with /proc/sys/vm/lowmem_reserve_ratio but must
> likely some defaults need tweaking.

Hmmm, after a bit of googling and a download of 2.6.18, it seems that
documentation on lowmem_reserve_ratio is still on the todo list.

cat /proc/sys/vm/lowmem_reserve_ratio 

gives me "256 256 32" on the system in question.  Can someone give me a
quick rundown of what this means?

Thanks,
Steve Bergman