Schedule FullGC with DisableExplicitGC enabled

Question

So the problem is that I would like to disable explicit GC to make sure no library can "force" FullGC with System.gc(). But I would like to still retain some control of when FullGC will happen. Is this doable? I mean is it possible to force (or strongly suggest) FullGC via JMX or some Tomcat management tools? Or maybe some command-line in cron?

The motivation of forcing FullGC is to make sure it doesn't happen in the middle of the day.

To my understanding FullGC will happen randomly with -XX:+DisableExplicitGC option. It will happen when Java will feel there is need to do this.

Or maybe there is some way to suggest Java something like "don't do FullGC from 7:00 to 16:00"?

Answer 1

Full GC can be forced even with -XX:+DisableExplicitGC by invoking a heap inspection operation like GC.class_histogram

Via JMX

MBean:     com.sun.management:type=DiagnosticCommand
Operation: gcClassHistogram

From the command line

jcmd <pid> GC.class_histogram

Patching System.gc

Alternatively, you can intercept System.gc() calls by preloading a shared library with a custom JVM_GC implementation.

Here is an example:

#define _GNU_SOURCE
#include <time.h>
#include <dlfcn.h>

void JVM_GC() {
    time_t timestamp = time(NULL);
    struct tm t;
    localtime_r(&timestamp, &t);

    // Don't do Full GC between 7:00 and 16:00
    if (t.tm_hour >= 7 && t.tm_hour < 16) {
        return;
    }

    void* original_jvm_gc = dlsym(RTLD_NEXT, "JVM_GC");
    if (original_jvm_gc != NULL) {
        ((void (*)()) original_jvm_gc)();
    }
}

Compile:

gcc -O2 -fPIC -shared -o librestrictgc.so restrictgc.c -ldl

Run:

LD_PRELOAD=/path/to/librestrictgc.so java <args>