When monitoring the memory usage of processes and zones, the numbers may not seem to add up, which can lead to confusion. This topic explains how SmartOS calculates the various values and why they normally do not agree.
In this topic:
The RSS values for the ps and prstat process lists come from the /proc file system. Internally these values are calculated by the rm_asrss() function, which determines how much memory the address space of a process is using. The comment on the rm_asrss() implementation explains the drawbacks to this approach:
This is currently implemented as the number of active hardware translations that have page structures. Therefore, it can underestimate the traditional resident set size, eg, if the physical page is present and the hardware translation is missing; and it can overestimate the rss, eg, if there are active translations to a frame buffer with page structs. Also, it does not take sharing and XHATs into account.
In addition to the limitations inherent in rm_asrss(), you can't simply sum the per-process RSS values to obtain the size of the zone's RSS. This is because it's common for processes in a zone to share pages, such as code pages from libc or other libraries on the system. Because of this type of page sharing, it is difficult to calculate the overall RSS for a zone.
Operationally, the overall RSS for a zone is the value reported by prstat -Z. This value is calculated by the internal getvmusage() system call, which counts the number of resident memory pages used by all of the processes in the zone. It tries to determine a more accurate zone RSS value by avoiding double-counting pages. This is the value used for memory capping.
The getvmusage() system call calculates RSS so that for a given zone, a page is only counted once. For example, if multiple processes in the same zone map the same page, then the zone is only charged once for that page. On the other hand, if two processes in different zones map the same page, then both zones are charged for the page.
The following pseudocode describes how getvmusage() calculates the RSS for a zone.
Because both the /proc and the getvmusage() RSS calculation are looking at processes at a point in time, it is possible for the numbers to disagree or to be different from one moment to the next as the processes on the system cause page faults, as pages are paged out, or as processes start and end.
Thus, the RSS is at best an approximation of the physical memory usage of processes and zones.