large pages (amd64)

[Alan Cox]

On Sun, Jun 28, 2009 at 12:36 PM, Wojciech Puchar <
wojtek at wojtek.tensor.gdynia.pl> wrote:

> i enabled
> vm.pmap.pg_ps_enabled: 1
>
>
> could you please explain what exactly this values means?
> because i don't understand why promotions-demotions!=mappings
>

"mappings" is not what you seem to think it is.  vm.pmap.pde.mappings is the
number of 2/4MB page mappings that are created directly and not through the
incremental promotion process.  For example, it counts the 2/4MB page
mappings that are created when the text segment of a large executable, e.g.,
gcc, is pre-faulted at startup or when a graphics card's frame buffer is
mmap()ed.

Moreover, not every promoted mapping is demoted.  A promoted mapping may be
destroyed without demotion, for example, when a process exits.  This is, in
fact, the ideal case because it is cheaper to destroy a single 2/4MB page
mapping instead of 512 or 1024 4KB page mappings.


>
> vm.pmap.pde.promotions: 2703
> vm.pmap.pde.p_failures: 6290
> vm.pmap.pde.mappings: 610
> vm.pmap.pde.demotions: 289
>
>
>
>
> other question - tried enabling it on my i386 laptop (256 megs ram), always
> mappings==0, while promitions>demotions>0.
>

The default starting address for executables on i386 is not aligned to a
2/4MB page boundary.  Hence, "mappings" are much less likely to occur.


> certainly there are apps that could be put on big pages, gimp editing 40MB
> bitmap for example


Regards,
Alan

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