For some reason, we did not implement SystemInfo::getPhysicalRam() on Mac OS. It returns 0 on that platform (and also on FreeBSD, incidentally).

The only consumer of this function is DocMemMonitor() in aggregation framework. It seems to be trying to limit how much physical RAM an operation will take, and uasserts if we exceed some threshold.

The comment for getPhysicalRam() says:

This should only be used for "advisory" purposes, and not as a hard
value, because this could be deceptive on virtual hosts, and because
this will return zero on platforms that do not support it.

So it sounds like we shouldn't be using this function to abort operations in aggregation. In fact, maybe we should just get rid of the function altogether, since no one else is using it.