Changes

User pages is the second importaint important resource (after [[kmemsize]]) which must be accounted.Despite Unlike kernel memory, which is either used or not, the set of user pages may have different classifications. Pages may be backed by file, locked, unused, i.e. requesed requested with <code>mmap</code>/<code>brk </code> but not yet touched and so on. Thus user pages accounting is trickier that than the one for kernel pages' one.

There are tree ways of approaches to user pages' accoutingaccounting.

; Account all the mappings on mmap/brk and reject as soon as the sum of VMA-'s lenghts lengths reaches the barrier.: This type approach is very bad as applications ''always'' map more than they really use and , very oftem often MUCH more.; Account only the really used memory and reject as soon as RSS <ref>{{H:title|Resident Set Size</ref> |RSS}} reaches the limit.: This type approach is not good either as the only place where pages appear in user space is page fault handler and the only way to reject is killing the task. Comparing to previous scenarion this is much worse as application won't even be able to close correctly.

# ''shmem mapping'' - this is the — a mapping of file, that belong belonging to <code>tmpfs</code>. UBC accounts these pages separately and the description below doesn't take them such pages into account;# ''private mapping'' - — this includes the following types of mappings:

#: Both types are not backed by disk file and thus may not be just freed. These mappings are charged with possible reject right when they are made - — in <code>sys_mmap()</code>/<code>sys_brk()</code>.# ''unused pages'' - — this is the number of pages which belong to private mapping, but are not yet touched.

* <math>UB_{privvm}</math> is the total number of privvmpages accounted on UB. In other words, this is the value seen in <code>/proc/user_beancounters</code> in <code>privvmpages.held</code>;* <math>UB_{unused}</math> is the total number of unused pages. This parameter is shown in <code>/proc/user_beancounters_debug</code> file;* <math>Frac(page, UB)</math> is some value that represents the part of the page charged to beancounter in case when page is mapped;* <math>RSS</math> is the amount of physical memory used by processes. Page fraction (— <math>Frac(page, UB)</math>) — normally should be <math>\frac{1}{N}</math>, where <math>N</math> is the number of UBs the pages is are shared between, but this is bad as since adding a new UB to page shared set would require recalculating recalculation of the whole current set. In UB <math>Frac(page, UB) = \frac{1}{2^{UB_{shift}}}</math>, where <math>UB_{shift}</math> is some parameter which is calculated so that

The notation <math>page \in UB</math> means "“<em>there exists an mm_struct, where the page <math>page</math> is mapped to and this mm_struct belongs to <math>UB</math> beancounter</em>"”. This type of calculation allowed to make allows making an O(1) algorithm of fractions accounting.

<tt>[[Privvmpages</tt> ]] accounts the sum of unused pages and the "normalized" “normalized” number of rss RSS pages:

If you summ sum the <tt>[[privvmpages</tt> ]] of all beancounters in the system you'll get an upper estimation of current physical memory usage by processes.

\sum _{UB} UB_{privvm} =

To get ''real'' physical memory usage <tt>[[physpages</tt> ]] resource is used <ref>Actually <tt>physpages</tt> also include tmpfs resident pages</ref>.