Selective swap out of processes

["Javier Cabezas Rodríguez" <jcabezas@ac.upc.edu>]

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Hi all,

I am trying to reduce the main memory power consumption when the system
is idle. In order to achieve it, I want to freeze some processes
(user-defined) when the system enters a long idle period and swap them
out to the disk. After that, more memory is free and then, the remaining
used memory can be moved to a minimal set of memory ranks so the rest of
ranks can be switched off.

To the best of my knowledge, a process can own the following types of
memory pages:
- Mapped pages
        · Executable and Read-only mapped pages that are backed by a
        file in the disk. These pages can be directly unmapped (if they
        are not shared) -> UNMAP
        · Writable file mapped pages that must be flushed to disk
        (synced) before they are unmapped -> SYNC + UNMAP
- Anonymous pages in User Mode address spaces -> SWAP
-  Mapped pages of tmpfs filesystem -> SWAP

I have implemented the process selection mechanism (using an entry for
each PID in proc), and the process freezing/resume (using the
refrigerator function, like in the hibernation code).

Now I am implementing the memory freeing. The biggest problem here is
that the regular swapping out algorithm of the kernel only frees memory
when it is needed, so I don't know which is the behaviour of the
standard routines in this situation.  I have looked at the standard
swapping functions (shrink_zones, shrink_zone, ...) and I think they
handle all the  process page types I enumerated previously. So, for each
VMA of the process,  I build a page list with all the pages and pass it
as a parameter to shrink_page_list (before that I remove them from the
LRU active/inactive lists with del_page_from_lru).

First I have tried with the executable VMA (of a lynx process) mapped to
the executable file. However none of the pages is freed.
shrink_page_list skips each page due to this check:

referenced = page_referenced(page, 1);
/* In active use or really unfreeable?  Activate it. */
if (referenced && page_mapping_inuse(page))
	goto activate_locked;

It seems they are mapped somewhere else and they cannot be freed. So,
which operations should I perform on the pages (try_to_unmap,
pte_mkold, ...) before I call shrink_page_list?

I would be eternally grateful if someone could help me with this :-)

Thanks in advance.

Javi

-- 
Javier Cabezas Rodríguez
Phd. Student - DAC (UPC)
jcabezas@ac.upc.edu

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