This monitors the available virtual memory space of the computer (commonly known as "swap space"),
and if it goes below a threshold, the slave is marked offline.
The exact definition of the swap space is platform dependent.
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On Windows, this value is
the available space in the page file,
but because Windows is capable of expanding the page file size automatically, the value doesn't really mean much.
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On Linux, this value is obtained from /proc/meminfo.
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On other Unix systems, this value is obtained by executing the top command.
If you have an operating system where this value is not reported, please let us know so that we can improve this.