Improving the Ruby GC

overview

This page details some changes to the ruby garbage collector which seem to afford a 25% reduction in maximum heap memory usage, and nearly double the amount of heap space ruby's is able to reclaim. This comes at the cost of a 2% performance hit. More to come, stay tuned.

patch

The latest patch to ruby 1.8.6 is available here.

summary of changes

• Fixed 2k heaps
• Heap implementation broken into separate c/h file
• heaps stored sorted by address for efficient is_pointer_to_heap()
• "adaptive GC frequency" - perform GC more frequently when free percentage is high.

testing strategy

To test changes, I'm comparing the performance of a handful of tests (most contributed by Hugh Sasse, many thanks!). Ruby 1.8.6 unpatched is run over these tests, then compared against the performance of ruby 1.8.6 with my patch. In order to get "maximum heap size", "current heap size", and "nmber of gc passes", a GC.heap_info function has been hacked in to both.

NOTE: When referring to heap size, we're really talking about the amount of memory dedicated to ruby heaps, not the total heap usage of the ruby process.

finally, all tests are performed on a macbook dual core with 2 gigs of RAM. We use wallclock time, but I've ensured an idle machine and rerun tests to verify that system load doesn't significantly affect results.

data

test	before				after				difference
test	time	gc passes	max mem	final mem	time	gc passes	max mem	final mem	time	gc passes %	max mem %	final mem %
cases/weak_hashes_read.rb	0.459784	8	4473980	4473980	0.448108	9	2570240	2570240	97%	112%	57%	57%
cases/create_hashes_yaml.rb	25.913408	75	27300060	8253120	26.433873	75	18012160	614400	102%	100%	65%	7%
cases/create_hashes.rb	0.134596	4	1208000	560000	0.142204	6	1077248	75776	105%	150%	89%	13%
cases/arrays_read_yaml.rb	0.76034	14	4473980	4473980	0.752775	13	3278848	3278848	99%	92%	73%	73%
cases/ostruct_read.rb	2.64286	19	15055440	15055440	2.865675	22	8781824	8781824	108%	115%	58%	58%
cases/create_ostructs_yaml.rb	13.673743	55	27299780	15055440	14.471611	57	15616000	9125888	105%	103%	57%	60%
cases/classes_read_yaml.rb	0.825455	13	4473940	4473940	0.84994	13	4151296	4149248	102%	100%	92%	92%
cases/create_weak_hashes2.rb	0.497352	7	4473940	4473940	0.512148	9	2643968	2490368	102%	128%	59%	55%
cases/create_arrays_yaml.rb	9.002692	43	15055660	8253140	9.168935	44	9859072	2252800	101%	102%	65%	27%
cases/create_arrays.rb	0.170999	4	2374420	560000	0.17948	6	1912832	75776	104%	150%	80%	13%
cases/growarray.rb	1.933119	9	200000	200000	2.116156	9	102400	75776	109%	100%	51%	37%
cases/arrays_read.rb	0.089326	4	2374460	560000	0.092244	5	1679360	73728	103%	125%	70%	13%
cases/hashes_read_yaml.rb	1.982008	23	4473920	4473920	1.908458	20	4532224	4532224	96%	86%	101%	101%
cases/plist.rb	4.926278	23	15055760	15055760	4.773025	18	12734464	10463232	96%	78%	84%	69%
cases/create_ostructs.rb	3.076644	22	15055660	11276480	3.284315	24	8800256	92160	106%	109%	58%	0%
cases/create_weak_hashes.rb	1.205506	17	4473960	4473960	1.131734	14	5253120	4933632	93%	82%	117%	110%
cases/classes_read.rb	0.056913	3	1208000	1208000	0.062934	5	919552	919552	110%	166%	76%	76%
cases/shrinkarray.rb	1.932001	9	200000	200000	2.122906	9	100352	77824	109%	100%	50%	38%
cases/ostruct_read_yaml.rb	1.142735	20	2374400	2374400	1.114133	17	3178496	3178496	97%	85%	133%	133%
cases/hashes_read.rb	0.0522	3	1208000	1208000	0.052393	4	876544	876544	100%	133%	72%	72%
									102%	110%	75%	55%

feedback

send perf feedback to lloyd at hilaiel d0t com.