memcapper

capping

The physical memory limit and the swap cap are two independent and 
unrelated limits.

The physical memory limit is a soft cap that controls how many pages of
physical memory can be resident for the zone. Once that value is exceeded 
the memory capper will invalidate pages until the RSS of the set of
processes within the zone is below the limit. 
 
The swap cap is a hard limit and is used to control how much anonymous
memory is available. Because anonymous memory is reserved when it is
requested, it is easy to enforce a hard cap.

The reason that the physical cap is a soft limit is that each physical 
memory page for a process is backed by something. This can be program text 
files, other files that are mmap-ped or anonymous memory (heap and stack) 
which is backed by swapfs (and thus is the only memory covered by the swap
cap). Because memory is consumed by demand paging, whenever a process touchs 
a page which is not resident the data is brought into memory from the 
backing store. Likewise, if the VM system needs a page for something else 
it is free to reclaim a page from the process by invalidating its mapping
(after writing the page to backing store if it is dirty). Because many 
non-anonymous mappings are shared among more than one process (e.g. libc)
there is no simple way to assign ownership of a page to an individual process. 
Thus, you can see why the physical limit is a soft cap.

To implement the physical limit the memory capper invalidates page mappings
for an invidual process although, as described above, this does not impact 
other processes which are also referencing the page. The general intention 
of the physical limit is to prevent the VM system from forcing pages out from 
other processes due to a greedy process touching lots of pages. If the capper 
is disabled then a process can have as much RSS as the VM system allows. 
There is also nothing inherently wrong here and the VM system will simply 
demand page in pages for other processes  again when they are touched. The 
worst effect is that other processes in other zones might experience more 
paging activity than is fair.