cs: scale GC trigger by number of running places

Since all places share the same pool of memory, scale the
amount of memory allowed before trigger a (minor) GC by
the number of running processes.

racket/src/cs/rumble/memory.ss

@@ -32,6 +32,12 @@
 (define (set-reachable-size-increments-callback! proc)
   (set! reachable-size-increments-callback proc))
 
+(define allocating-places 1)
+(define (collect-trip-for-allocating-places! delta)
+  (with-global-lock
+   (set! allocating-places (+ allocating-places delta))
+   (collect-trip-bytes (* allocating-places (* 8 1024 1024)))))
+
 ;; Replicate the counting that `(collect)` would do
 ;; so that we can report a generation to the notification
 ;; callback

racket/src/cs/rumble/place.ss

@@ -81,6 +81,7 @@
   (define (fork-place thunk finish-proc)
     (do-prepare-for-place)
     (fork-thread (lambda ()
+                   (collect-trip-for-allocating-places! +1)
                    (init-virtual-registers)
                    (place-registers (vector-copy place-register-inits))
                    (root-thread-cell-values (make-empty-thread-cell-values))
@@ -93,6 +94,7 @@
                                     (thunk)
                                     0))])
                      (finish-proc result)
+                     (collect-trip-for-allocating-places! -1)
                      (do-destroy-place)))))
   ;; Must be called within an engine, used for memory accounting:
   (define (current-place-roots)