On this page:

15.4 Garbage Collection

Forces an immediate garbage collection. Some effectively unreachable data may remain uncollected, because the collector cannot prove that it is unreachable.

The collect-garbage procedure provides some control over the timing of collections, but garbage will obviously be collected even if this procedure is never called.

(current-memory-use [cust])  exact-nonnegative-integer?
  cust : custodian? = #f
Returns an estimate of the number of bytes of memory occupied by reachable data from cust. (The estimate is calculated without performing an immediate garbage collection; performing a collection generally decreases the number returned by current-memory-use.) If cust is not provided, the estimate is a total reachable from any custodians.

When Racket is compiled without support for memory accounting, the estimate is the same (i.e., all memory) for any individual custodian; see also custodian-memory-accounting-available?.

Dumps information about memory usage to the (low-level) standard output port.