Understanding page statistics

Understanding page statistics

[Martin Ichilevici de Oliveira]

Hello,

I'm starting to study and learn the kernel, and I'm particulary
interested in the virtual memory mechanism and it's application in NUMA
architectures.

As far as I could understand, the kernel keeps track of some
information/statistics about each page, and I'd like to read them with a
kernel module - just to see what kind of information is available
(something more detailed than /proc/<pid>/maps).

In a LWN article [1], I read the following:

"The scheduler will periodically scan through each process's address
space, revoking all access permissions to the pages that are currently
resident in RAM. The next time the affected process tries to access that
memory, a page fault will result. The scheduler will trap that fault and
restore access to the page in question; it will also increment an access
counter in a per-process array indexed by the NUMA node number."

I've built a simple kernel module that, given a PID and virtual address,
retrieves the corresponding struct page. I'm now unsure now where to look 
for such statistics and counters. Any hints on relevant data structures are
appreciated.

Thank you,
Martin

[1] : http://lwn.net/Articles/568870/
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Understanding page statistics

[Steffen Vogel]

Am Mittwoch, den 13.08.2014, 18:31 -0300 schrieb Martin Ichilevici de
Oliveira:
> "The scheduler will periodically scan through each process's address
> space, revoking all access permissions to the pages that are currently
> resident in RAM. The next time the affected process tries to access that
> memory, a page fault will result.

I don't know how often the flags are reset.
But isn't this a huge overhead? I don't see any advantages for this on
UMA machines.

> I've built a simple kernel module that, given a PID and virtual address,
> retrieves the corresponding struct page. I'm now unsure now where to look 
> for such statistics and counters. Any hints on relevant data structures are
> appreciated.

Oh okay, If I'm correct 'struct page' represents a physical page frame
rather than virtual mapped page (see [2]).

The first thing which comes in my mind is LRU aka the kernels page frame
reclamation which is used for swapping unused pages to disk.
I'm mot really sure wheather the LRU lists are based on physical pages.
But I assume so.

You might want to have a look at /proc/pid/pagemap for detailed
information of the VA per process [1].
Theres also a /proc/kpageflags and /proc/kpagecount which contain
information over physical pages [1].


Steffen

[1] https://www.kernel.org/doc/Documentation/vm/pagemap.txt
[2] http://www.tldp.org/LDP/tlk/ds/ds.html
[3] http://linux-mm.org/LRU
[4] https://www.kernel.org/doc/gorman/html/understand/understand013.html

-- 
Steffen Vogel
Robensstra?e 69
52070 Aachen

Mail: post at steffenvogel.de
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