Interface VirtualMemory

All Known Implementing Classes:
AbstractVirtualMemory, FreeBsdVirtualMemory, MacVirtualMemory, OpenBsdVirtualMemory, SolarisVirtualMemory
@PublicApi @ThreadSafe public interface VirtualMemory
The VirtualMemory class tracks information about the use of a computer's virtual memory (swap file) which temporarily moves rarely accessed information to a disk or other storage device.

Operating systems differ significantly in how they manage virtual memory:

  • Windows uses a commit-charge model: every virtual memory allocation must be backed by physical RAM plus the pagefile. Allocations that would exceed this commit limit fail immediately, so the values from getVirtualMax() and getVirtualInUse() reflect guaranteed (reserved) capacity.
  • Linux uses overcommit by default: the kernel allows processes to allocate more virtual memory than is physically available, on the assumption that not all allocated memory will be used simultaneously. If memory is actually exhausted, the Out-Of-Memory (OOM) killer terminates processes to reclaim memory. The vm.overcommit_memory sysctl controls this behavior (0 = heuristic overcommit, 1 = always overcommit, 2 = strict commit accounting similar to Windows). The getVirtualMax() value reflects the kernel's CommitLimit.
  • macOS uses a compressed-memory approach, compressing inactive pages in RAM before resorting to swap on disk. Swap usage on macOS tends to be lower than on other platforms because compression reclaims significant memory without disk I/O.
These differences affect how to interpret the values returned by this interface, particularly getVirtualMax() and getVirtualInUse().

  • Method Summary

    Modifier and Type
    Method
    Description
    long
    getSwapPagesIn()
    Number of pages read from paging/swap file(s) to resolve hard page faults.
    long
    getSwapPagesOut()
    Number of pages written to paging/swap file(s) to free up space in physical memory.
    long
    getSwapTotal()
    The current size of the paging/swap file(s), in bytes.
    long
    getSwapUsed()
    The current memory committed to the paging/swap file(s), in bytes
    long
    getVirtualInUse()
    The memory currently committed by the system, in bytes.
    long
    getVirtualMax()
    The maximum memory that can be committed by the system without extending the paging file(s), in bytes.

  • Method Details

    • getSwapTotal

      long getSwapTotal()
      The current size of the paging/swap file(s), in bytes. If the paging/swap file can be extended, this is a soft limit.
      Returns:
      Total swap in bytes.

    • getSwapUsed

      long getSwapUsed()
      The current memory committed to the paging/swap file(s), in bytes
      Returns:
      Swap used in bytes

    • getVirtualMax

      long getVirtualMax()
      The maximum memory that can be committed by the system without extending the paging file(s), in bytes. Also called the Commit Limit. If the paging/swap file can be extended, this is a soft limit. This is generally equal to the sum of the sizes of physical memory and paging/swap file(s).

      On Linux, represents the total amount of memory currently available to be allocated on the system based on the overcommit ratio, identified as CommitLimit. This may be higher or lower than the total size of physical and swap memory depending on system configuration.

      Returns:
      Max Virtual Memory in bytes

    • getVirtualInUse

      long getVirtualInUse()
      The memory currently committed by the system, in bytes. Also called the Commit Total. This is generally equal to the sum of the bytes used of physical memory and paging/swap file(s).

      On Windows, committing pages changes this value immediately; however, the physical memory is not charged until the pages are accessed, so this value may exceed the sum of used physical and paging/swap file memory.

      Returns:
      Swap used in bytes

    • getSwapPagesIn

      long getSwapPagesIn()
      Number of pages read from paging/swap file(s) to resolve hard page faults. (Hard page faults occur when a process requires code or data that is not in its working set or elsewhere in physical memory, and must be retrieved from disk.) This property was designed as a primary indicator of the kinds of faults that cause system-wide delays. It includes pages retrieved to satisfy faults in the file system cache (usually requested by applications) and in non-cached mapped memory files.
      Returns:
      Pages swapped in

    • getSwapPagesOut

      long getSwapPagesOut()
      Number of pages written to paging/swap file(s) to free up space in physical memory. Pages are written back to disk only if they are changed in physical memory, so they are likely to hold data, not code. A high rate of pages output might indicate a memory shortage. The operating system writes more pages back to disk to free up space when physical memory is in short supply.
      Returns:
      Pages swapped out