On a 64-bit system, setting bytes-consed-between-gcs to 9GB results in a nursery memory usage pattern with a 1GB range.
The setter & reader for bytes-consed-between-gcs in gc.lisp uses (sb!alien:unsigned 32), while the C code is an unsigned long. This prevents 64-bit systems from using >4GB nursery size, which is sometimes desirable. It seems to silently mask the 64-bit number to 32-bit without any warnings or other bugs.
Changing the accessors to use sb!alien:unsigned-long seemed to fix it on our end. The current git browser on SF shows unsigned 32's.