The garbage collector distributed with ruby tries to adapt memory
usage to the amount of memory used by the program, dynamically growing
or shrinking the allocated heap as it sees fit.

For long running server apps this doesn't really work very well. The
performance very much depends on the ratio heap size/program size. It
behaves somewhat erratic: adding code can actually make your program
run faster.

The supplied patch fixes this problem: it allows one to specify the
initial heap size to start with. Heap size will never drop below the
initial size. By carefully selecting the initial heap size one can
increase startup time and throughput of server apps.

The patch is for 1.8.4, but applies to and works with 1.8.2 as well.

Heap size and other options are controlled through environment
variables:

RUBY_HEAP_MIN_SLOTS
    - the initial heap size in number of slots used

RUBY_GC_MALLOC_LIMIT
    - the amount of C data structures which can be allocated
      without triggering a GC (if this is set too low, GC will be
      started even if there are empty slots available)

RUBY_HEAP_FREE_MIN
    - number of free slots that should be available after GC
      if fewer slots are available, additional heap will be allocated

The following values make the patched GC behave exactly like the
unpatched GC:

RUBY_HEAP_MIN_SLOTS=10000
RUBY_GC_MALLOC_LIMIT=8000000
RUBY_HEAP_FREE_MIN=4096
                        
I have found the following values to be suitable for my rails app:

RUBY_HEAP_MIN_SLOTS=600000
RUBY_GC_MALLOC_LIMIT=60000000
RUBY_HEAP_FREE_MIN=20000

You can watch memory usage of the ruby interpreter by setting
RUBY_GC_STATS=1, before you invoke perf_bench.

Per default, GC data gets written to stderr. You can change this
behavior by setting environment variable RUBY_GC_DATA_FILE.

Additionally, the Ruby module GC gets some new methods:

GC.enable_stats   - enable GC statistics collection
GC.disable_stats  - disable GC statistics collection
GC.clear_stats    - reset GC statistics
GC.collections    - number of collections since stats have been enabled
GC.time           - GC time used since stats have been enabled (miro seconds)
GC.dump           - dumps current heap topology to the GC log file
GC.log            - log given string to the GC log file

railsbench detects whether the patch has been applied and provides
GC statistics if available.