Editing User pages accounting

{{UBC toc}}

This article describes internals of user-space pages accounting.

== Introduction ==
User pages is the second important resource (after [[kmemsize]]) which must be accounted.
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. requested with <code>mmap</code>/<code>brk</code> but not yet touched and so on. Thus user pages accounting is trickier than the one for kernel pages.

== Ways of accounting ==
There are different approaches to user pages control:

; Account all the mappings on mmap/brk and reject as soon as the sum of VMA's lengths reaches the barrier.
: This approach is very bad as applications ''always'' map more than they really use, very often MUCH more.
; Account only the really used memory and reject as soon as {{H:title|Resident Set Size|RSS}} reaches the limit.
: This 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 terminate gracefully.
; Account a part of memory on mmap/brk and reject there, and account the rest of the memory in page fault handlers without any rejects.
: This type of accounting is used in UBC.
; Account physical memory and behave like a standalone kernel - reclaim user memory when run out of it.
: This type of memory control is to be introduced later as an addition to current scheme. UBC provides all the needed statistics for this (physical memory, swap pages etc.)

== UBC user pages accounting ==
=== Terms ===
The following terms are used by UBC:
* ''shmem mapping'' — a mapping of file belonging to <code>tmpfs</code>. UBC accounts these pages separately and the description below doesn't take such pages into account;
* ''private mapping'' — this includes the following types of mappings:
** writable anonymous mappings
** writable private file 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 model ===
The following notations are used:

* <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 are shared between, but this is bad since adding a new UB to page shared set would require 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
<center>
<math>\sum _{UB : page \in UB} Frac(page, UB) = 1, \forall page</math>.
</center>
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 allows making an O(1) algorithm of fractions accounting.

[[Privvmpages]] accounts the sum of unused pages and the “normalized” number of RSS pages:

<center>
<math>
UB_{privvm} = UB_{unused} + \sum _{page \in UB} Frac(page, UB)
</math>
</center>

If you sum the [[privvmpages]] of all beancounters in the system you'll get an upper estimation of current physical memory usage by processes:

<center>
<math>
\sum _{UB} UB_{privvm} = \,
</math>

<math>
\sum _{UB} UB_{unused} + \sum _{UB} {\sum _{page \in UB} Frac(page, UB)} =
</math>

<math>
\sum _{UB} UB_{unused} + \sum _{page} \sum _{UB: page \in UB} Frac(page, UB) =
</math>

<math>
\sum _{page: page\ is\ mapped} 1 + \sum _{UB} UB_{unused} =
</math>

<math>
RSS + \sum _{UB} UB_{unused} \,
</math>
</center>

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

<center>
<math>
UB_{physpages} = \sum _{page \in UB} Frac(page, UB)
</math>
</center>
Thus
<center>
<math>
\sum _{UB} UB_{physpages} = RSS \,
</math>
</center>

----
<references/>

[[Category: Kernel internals]]