Update docs

doc/collector-pcc.md

@@ -20,8 +20,11 @@ the same performance characteristics with a single mutator and with
 parallelism disabled, additionally allowing multiple mutators, and
 scaling better with multiple tracing threads.
 
-Also like `semi`, `pcc` is not generational yet.  If and when `pcc`
-grows a young generation, it would be a great collector.
+`pcc` has a generational configuration, conventionally referred to as
+`generational-pcc`, in which both the nursery and the old generation are
+copy spaces.  Objects stay in the nursery for one cycle before moving on
+to the old generation.  This configuration is a bit new (January 2025)
+and still needs some tuning.
 
 ## Implementation notes
 

doc/collector-semi.md

@@ -19,5 +19,5 @@ size, and performs best with ample heap sizes; between 3× and 5× is
 best.
 
 The semi-space collector doesn't support multiple mutator threads.  If
-you want a whole-heap copying collector for a multi-threaded mutator,
-look at [pcc](./collector-pcc.md).
+you want a copying collector for a multi-threaded mutator, look at
+[pcc](./collector-pcc.md).

doc/collectors.md

@@ -3,8 +3,9 @@
 Whippet has four collectors currently:
  - [Semi-space collector (`semi`)](./collector-semi.md): For
    single-threaded embedders who are not too tight on memory.
- - [Parallel copying collector (`pcc`)](./collector-pcc.md): Like `semi`,
-   but with support for multiple mutator and tracing threads.
+ - [Parallel copying collector (`pcc`)](./collector-pcc.md): Like
+   `semi`, but with support for multiple mutator and tracing threads and
+   generational collection.
  - [Mostly marking collector (`mmc`)](./collector-mmc.md):
    Immix-inspired collector.  Optionally parallel, conservative (stack
    and/or heap), and/or generational.
@@ -30,8 +31,9 @@ precise roots, then go for `stack-conservative-parallel-mmc` directly.
 
 ## More collectors
 
-It would be nice to have a classic generational GC, perhaps using
+It would be nice to have a generational GC that uses the space from
 `parallel-mmc` for the old generation but a pcc-style copying nursery.
+We have `generational-pcc` now, so this should be possible.
 
 Support for concurrent marking in `mmc` would be good as well, perhaps
 with a SATB barrier.  (Or, if you are the sort of person to bet on

doc/manual.md

@@ -112,8 +112,8 @@ If the `gc_atomic_forward`'s state is `BUSY`, the collector will call
 `gc_atomic_forward_retry_busy`; a return value of 0 means the object is
 still busy, because another thread is attempting to forward it.
 Otherwise the forwarding state becomes either `FORWARDED`, if the other
-thread succeeded in forwarding it, or `ABORTED`, indicating that the
-other thread failed to forward it.
+thread succeeded in forwarding it, or go back to `NOT_FORWARDED`,
+indicating that the other thread failed to forward it.
 
 If the forwarding state is `FORWARDED`, the collector will call
 `gc_atomic_forward_address` to get the new address.
@@ -351,6 +351,12 @@ $(COMPILE) -DGC_PARALLEL=1 -DGC_PRECISE_ROOTS=1 \
   -include foo-embedder.h -o gc.o -c pcc.c
 ```
 
+You can also build `pcc` in a generational configuration by passing
+`-DGC_GENERATIONAL=1`.  The nursery is 2 MB per active mutator, capped
+to the number of processors, so if the last cycle had a maximum of 4
+mutator threads active at the same time and your machine has 24 cores,
+your nursery would be 8 MB.
+
 #### Building `mmc`
 
 Finally, there is the mostly-marking collector.  It can collect roots