Cache Control

The virtual memory system in SunOS is a cache system, in which processor memory buffers data from file system objects. Interfaces are provided to control or interrogate the status of the cache.

Using mincore

The mincore(2) interface determines the residency of the memory pages in the address space covered by mappings in the specified range. Because the status of a page can change after mincore checks the page but before mincore returns the data, returned information can be outdated. Only locked pages are guaranteed to remain in memory.

Using mlock and munlock

mlock(3C) causes the pages in the specified address range to be locked in physical memory. References to locked pages in this process or in other processes do not result in page faults that require an I/O operation. Because this I/O operation interferes with normal operation of virtual memory, as well as slowing other processes, the use of mlock is limited to the superuser. The limit to the number of pages that can be locked in memory is dependent on system configuration. The call to mlock fails if this limit is exceeded.

munlock releases the locks on physical pages. If multiple mlock calls are made on an address range of a single mapping, a single munlock call releases the locks. However, if different mappings to the same pages are locked by mlock, the pages are not unlocked until the locks on all the mappings are released.

Removing a mapping also releases locks, either through being replaced with an mmap(2) operation or removed with munmap(2).

The copy-on-write event that is associated with a MAP_PRIVATE mapping transfers a lock on the source page to the destination page. Thus locks on an address range that includes MAP_PRIVATE mappings are retained transparently along with the copy-on-write redirection. For a discussion of this redirection, see Creating and Using Mappings.

Using mlockall and munlockall

mlockall(3C) and munlockall(3C) are similar to mlock and munlock, but mlockall and munlockall operate on entire address spaces. mlockall sets locks on all pages in the address space and munlockall removes all locks on all pages in the address space, whether established by mlock or mlockall.

Using msync

msync(3C) causes all modified pages in the specified address range to be flushed to the objects mapped by those addresses. This command is similar to fsync(3C), which operates on files.