【转】Resin关于最小内存设置
resin.conf 中有 <min-free-memory>设置，是resin内部有一个thread每隔10秒钟检查JVM剩余内存，如果free memory小于 2*min-free-memory，则执行Runtime.gc() 强制进行GC，如果使用-XX:+DisableExplicitGC 禁用显式调用GC，则该设置无效。对应的代码如下（com/caucho/server/resin/Resin.java）：
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[*]long minFreeMemory = _server.getMinFreeMemory();
[*] if (minFreeMemory <= 0) {
[*] // memory check disabled
[*] }
[*] else if (2 * minFreeMemory < getFreeMemory(runtime)) {
[*] // plenty of free memory
[*] }
[*] else {
[*] if (log().isLoggable(Level.FINER)) {
[*] log().finer(L().l("free memory {0} max:{1} total:{2} free:{3}",
[*] "" + getFreeMemory(runtime),
[*] "" + runtime.maxMemory(),
[*] "" + runtime.totalMemory(),
[*] "" + runtime.freeMemory()));
[*] }
[*]
[*] log().info(L().l("Forcing GC due to low memory. {0} free bytes.",
[*] getFreeMemory(runtime)));
[*]
[*] runtime.gc();
[*]
[*] if (getFreeMemory(runtime) < minFreeMemory) {
[*] _isClosed = true;
[*] log().severe(L().l("Restarting due to low free memory. {0} free bytes",
[*] getFreeMemory(runtime)));
[*] return;
[*] }
其中getMinFreeMemory()如下：
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[*]private static long getFreeMemory(Runtime runtime)
[*] {
[*] long maxMemory = runtime.maxMemory();
[*] long totalMemory = runtime.totalMemory();
[*] long freeMemory = runtime.freeMemory();
[*] // Some JDKs (JRocket) return 0 for the maxMemory
[*] if (maxMemory < totalMemory)
[*] return freeMemory;
[*] else
[*] return maxMemory - totalMemory + freeMemory;
[*] }
计算jvm空余内存的方法值得注意一下：由于JVM可以使用-Xms 和-Xmx 指定启动内存和最大内存，所以计算空余内存的时候，需要考虑到JVM可以分配的最大内存（runtime.maxMemory()）和当前可使用的最大内存（runtime.totalMemory()），因此最终的free memory =当前空余内存（runtime.freeMemory()） + 可以分配但还没有分配的内存（runtime.maxMemory() - runtime.totalMemory()）
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