GC原因不一样，GC效果不一样

/art/runtime/gc/gc_cause.h

// What caused the GC?
 enum GcCause {
          // Invalid GC cause used as a placeholder.
          kGcCauseNone,
          // GC triggered by a failed allocation. Thread doing allocation is blocked waiting for GC before
          // retrying allocation.
          kGcCauseForAlloc,
          // A background GC trying to ensure there is free memory ahead of allocations.
          kGcCauseBackground,
          // An explicit System.gc() call.
          kGcCauseExplicit,
          // GC triggered for a native allocation when NativeAllocationGcWatermark is exceeded.
          // (This may be a blocking GC depending on whether we run a non-concurrent collector).
          kGcCauseForNativeAlloc,
          // GC triggered for a collector transition.
          kGcCauseCollectorTransition,
          // Not a real GC cause, used when we disable moving GC (currently for GetPrimitiveArrayCritical).
          kGcCauseDisableMovingGc,
          // Not a real GC cause, used when we trim the heap.
          kGcCauseTrim,
          // Not a real GC cause, used to implement exclusion between GC and instrumentation.
          kGcCauseInstrumentation,
          // Not a real GC cause, used to add or remove app image spaces.
          kGcCauseAddRemoveAppImageSpace,
          // Not a real GC cause, used to implement exclusion between GC and debugger.
          kGcCauseDebugger,
          // GC triggered for background transition when both foreground and background collector are CMS.
          kGcCauseHomogeneousSpaceCompact,
          // Class linker cause, used to guard filling art methods with special values.
          kGcCauseClassLinker,
          // Not a real GC cause, used to implement exclusion between code cache metadata and GC.
          kGcCauseJitCodeCache,
          // Not a real GC cause, used to add or remove system-weak holders.
          kGcCauseAddRemoveSystemWeakHolder,
          // Not a real GC cause, used to prevent hprof running in the middle of GC.
          kGcCauseHprof,
          // Not a real GC cause, used to prevent GetObjectsAllocated running in the middle of GC.
          kGcCauseGetObjectsAllocated,
          // GC cause for the profile saver.
          kGcCauseProfileSaver,
        };

常见GC类型

1.kGcCauseForAlloc：在分配内存时内存不够情况下引起的GC,这种情况下GC会stop World。Stop World 是由于并发GC时，其他线程都会停止。

Android中开发者可以用的GC类型

2.kGcCauseBackground：当内存达到一定阈值的时候引发GC，这个时候是一个后台GC,不会引起Stop World。系统低内存时候才用该处理方式

   private fun forceGC() {
        val myClass = classLoader.loadClass("com.android.internal.os.BinderInternal")
        val newInstanceMethod = myClass.getDeclaredMethod("forceGc",String::class.java)
        newInstanceMethod.isAccessible = true
        newInstanceMethod.invoke(null, "bg")
    }

3.kGcCauseExplicit：显示调用时进行的GC，如果ART打开了这个选项，在system.gc时会进行GC。System.gc() 或者 Runtime.getRuntime().gc();

4.如果有root权限， 执行adb shell kill -10 pid也可以调用GC， 在linux kill命令中状态10表示SIGUSR1，调用的是signal_catcher.cc的HandleSigUrs1函数。