0.9.8.19:

0.9.8.19:
        Changes to GENCGC memory zeroing behaviour that give a big
        performance boost, especially on modern processors.

        * Instead of zeroing memory by remapping memory with
          munmap/mmap at GC time, pages are just marked as needing
          zeroing and zeroed with memset when they're added to a new
          allocation region. This reduces GC latency both for the
          common and worst cases.

        * To keep the memory footprint down, clear the pages by
          remapping after major GCs (arbitrarily defined as a
          collection of generation 2 or older).  The memory freed from
          a minor GC is just going to get used again immediately , so
          releasing them back to the OS would make little sense.

        * Add a GENCGC mode (#define READ_PROTECT_FREE_PAGES) for
          catching attempts to read unallocated pages

        * See sbcl-devel "Changes to GENCGC memory zeroing" in 2005-12
          for more details and performance measurements. (Note
          that many parts of this patch have already been committed
          piecemeal over the last month, this is just the most
          significant chunk).

        * Performance effect on BSDs (which used a different zeroing
          strategy than Linux before this) is unknown.