Runtime.gc() calls do make a difference

Hi YH,

You said calling Runtime.gc() is counter-productive. My experiment shows otherwise. Below is a heavy java job that uses about 200MB memory. “JJ_1001001=233564416” means after processing 1001001 rows, 233MB memory was in use. As you can see, usage grows with data volume.

[2010-09-07 14:03:07,132 INFO .Main:39] – {before_JJ=17944080, JJ_1=149162416, JJ_100101=161278496, JJ_200201=177440888, JJ_300301=179908768, JJ_400401=182329728, JJ_500501=198315560, JJ_600601=200645064, JJ_700701=216799944, JJ_800801=218142816, JJ_900901=231948296, JJ_1001001=233564416, before_destroySingletons=143855904}

——– Now I added calls to Runtime.gc() after every 100,000 rows. Memory usage is now kept constant at around 120MB.

[2010-09-07 13:40:12,347 INFO .Main:39] – {before_JJ=17944080, JJ_1=120664072, JJ_100101=120664248, JJ_200201=120664072, JJ_300301=120664504, JJ_400401=120664176, JJ_500501=120664760, JJ_600601=120664584, JJ_700701=120664864, JJ_800801=120664840, JJ_900901=120665336, JJ_1001001=120665256, before_destroySingletons=131827256}

I run the job again. Results are similar, so GC behavior is not 100% predictable —

[2010-09-07 13:57:02,060 INFO .Main:39] – {before_JJ=17944080, JJ_1=120664224, JJ_100101=120663944, JJ_200201=120664528, JJ_300301=120664048, JJ_400401=120664632, JJ_500501=120664608, JJ_600601=120664888, JJ_700701=120664712, JJ_800801=120664992, JJ_900901=120665280, JJ_1001001=120665248, before_destroySingletons=131828080}

Here's how to request gc():

public static void runGC() {
// for whatever reason it helps to call Runtime.gc()
// using several method calls:
for (int r = 0; r < 4; ++r)

private static void _runGC() {
long usedMem1 = usedMemory(), usedMem2 = Long.MAX_VALUE;
for (int i = 0; (usedMem1 < usedMem2) && (i < 1000); ++i) {
usedMem2 = usedMem1;
usedMem1 = usedMemory();


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