NAME

swap — swap space

DESCRIPTION

The operating system uses demand paging as the primary mechanism to implement virtual memory. The system can also use traditional swapping, wherein an entire process state is moved between physical memory and swap space on disk, but it is very rare for the system to actually swap an entire process out to disk. A system which is swapping does not have enough enough physical memory to support the workload on the machine. In this case, work should be reduced or more memory added.

Given that the system rarely, if ever, swaps, but still has swap space configured, the question arises as to what this space is for?

In a demand paged virtual memory system, every page mapped by a process is backed by some object. For the actual program code used by the process, the backing objects are the underlying files in the filesystem for the program's binary, and any dynamically linked libraries. The portions of a process that are not backed by a named file, including its stack and its heap, are called anonymous memory. The system uses the swap space as the backing store for these pages. When the system determines that it needs to page out one of the anonymous pages, the swap space is where that page is written.

Unlike some other operating systems, illumos reserves the backing storage space for anonymous memory at the time of allocation. For example, if a process asks for more heap space, the total size of the swap allocation that may be required to store the new pages if they need to be paged out is reserved at that time. This does not mean that anything is written to the swap space, but simply that the space is reserved for the entire allocation. Thus, a process will always get a correct error from the sbrk(2) system call if swap space is unavailable. Some other operating systems don't allocate backing store for anonymous memory until it is used, so the error handling when space is not available can be complex or problematic on those systems.

The vmstat(8) command can be used to monitor swap and paging activity. The pmap(1) command can be used to inspect all of the mappings in a process address space, and their backing objects. The swap(8) command can be used to monitor, add and remove swap space.

The operating system provides the ‘zone.max-swap’ resource control to limit the amount of anonymous memory used by all of the processes within a zone. This resource control can also be configured under the capped-memory(2) setting for a zone. See the prctl(1) and zonecfg(8) man pages for information on setting this limit. The zone's usage against this resource control can be seen using the ‘swapresv_zone_{zoneid}’ kstat.

SEE ALSO

pmap(1), prctl(1), sbrk(2), resource_controls(7), kstat(8), swap(8), vmstat(8), zonecfg(8)