16.4 Garbage Collection

Set the PLTDISABLEGC environment variable (to any value) before Racket starts to disable garbage collection.

In Racket 3m (the main variant of Racket), each garbage collection logs a message (see Logging) at the 'debug level with topic 'GC. The data portion of the message is an instance of a gc-info prefab structure type with 10 fields as follows, but future versions of Racket may use a gc-info prefab structure with additional fields:

(struct gc-info (major? pre-amount pre-admin-amount code-amount

post-amount post-admin-amount

start-process-time end-process-time

start-time end-time)

#:prefab)

• The major? field indicates whether the collection was a “major” collection that inspects all memory or a “minor” collection that mostly inspects just recent allocations.

• The pre-amount field reports place-local memory use (i.e., not counting the memory use of child places) in bytes at the time that the garbage collection started.

• The pre-admin-amount is a larger number that includes memory use for the garbage collector’s overhead, such as space on memory pages that are mapped but not currently used.

• The code-amount field reports additional memory use for generated native code (which is the same just before and after a garbage collection, since it is released via finalization).

• The post-amount and post-admin-amount fields correspond to pre-amount and pre-admin-amount, but after garbage collection. In typical configurations, the difference between post-amount and pre-amount contributes to post-admin-amount, since reclaimed pages tend to stay in reserve with the expectation that they’ll be needed again (but the pages are released if multiple collections pass without need for the pages).

• The start-process-time and end-process-time fields report processor time (in the sense of current-process-milliseconds) at the start and end of garbage collection. The difference between the times is the processor time consumed by collection.

• The start-time and end-time fields report real time (in the sense of current-inexact-milliseconds) at the start and end of garbage collection. The difference between the times is the real time consumed by garbage collection.

The format of the logged message’s text is subject to change. Currently, after a prefix that indicates the place and collection mode, the text has the format

‹used›(‹admin›)[‹code›]; free ‹reclaimed›(‹adjust›) ‹elapsed› @ ‹timestamp›

‹used›		Collectable memory in use just prior to garbage collection
‹admin›		Additional memory used as to manage collectable memory
‹code›		Additional memory used for generated machine code
‹reclaimed›		Collectable memory reclaimed by garbage collection
‹adjust›		Negation of change to administrative memory minus ‹reclaimed›
‹elapsed›		Processor time used to perform garbage collection
‹timestamp›		Processor time since startup of garbage collection’s start

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 (unless garbage collection is disabled).

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?.

See also vector-set-performance-stats!.

Various combinations of v arguments can control the information in a dump. The information that is available depends on your Racket build; check the end of a dump from a particular build to see if it offers additional information; otherwise, all vs are ignored.