runtime/native_gc_triggering.md

Triggering the GC to reclaim non-Java memory
--------------------------------------------

Android applications and libraries commonly allocate "native" (i.e. C++) objects that are
effectively owned by a Java object, and reclaimed when the garbage collector determines that the
owning Java object is no longer reachable. Various mechanisms are used to accomplish this. These
include traditional Java finalizers, more modern Java `Cleaner`s, Android's `SystemCleaner` and,
for platform code, `NativeAllocationRegistry`. Internally, these all rely on the garbage
collector's processing of `java.lang.ref.Reference`s.

Historically, we have encountered issues when large volumes of such "native" objects are owned by
Java objects of significantly smaller size. The Java garbage collector normally decides when to
collect based on occupancy of the Java heap. It would not collect if there are few bytes of newly
allocated Java objects in the Java heap, even if they "own" large amounts of C++ memory, which
cannot be reclaimed until the Java objects are reclaimed.

This led to the development of the `VMRuntime.registerNativeAllocation` API, and eventually the
`NativeAllocationRegistry` API. Both of these allow the programmer to inform the ART GC that a
certain amount of C++ memory had been allocated, and could only be reclaimed as the result of a
Java GC.

This had the advantage that the GC would theoretically know exactly how much native memory was
owned by Java objects. However, its major problem was that registering the exact size of such
native objects frequently turned out to be impractical. Often a Java object does not just own a
single native object, but instead owns a complex native data structure composed of many objects in
the C++ heap. Their sizes commonly depended on prior computations by C++ code. Some of these might
be shared between Java objects, and could only be reclaimed when all of those Java objects became
unreachable. Often at least some of the native objects were allocated by third-party libraries
that did not make the sizes of its internal objects available to clients.

In extreme cases, underestimation of native object sizes could cause native memory use to be so
excessive that the device would become unstable. At one time, a particularly nasty arithmetic
expression would cause the Google Calculator app to allocate sufficiently large native arrays of
digits backing Java `BigInteger`s to force a restart of system processes. (This has since been
addressed in other ways as well.)

Thus we switched to a scheme in which sizes of native objects are commonly no longer directly
provided by clients. The GC instead occasionally calls `mallinfo()` to determine how much native
memory has been allocated, and assumes that any of this may need the collector's help to reclaim.
C++ memory allocated by means other than `malloc`/`new` and owned by Java objects should still be
explicitly registered. This loses information about Java ownership, but results in much more
accurate information about the total size of native objects, something that was previously very
difficult to approximate, even to within an order of magnitude.

The triggering heuristic
------------------------

We compute the total amount of native memory allocated as the sum of

1. memory allocated, but not yet deallocated, by the system memory allocator, as reported by
   `mallinfo()`, and

2. the number of bytes registered via `VMRuntime.registerNativeAllocation()` and not yet
   unregistered via `VMRuntime.registerNativeFree()`. This includes non-malloc-allocated objects
   allocated via `NativeAllocationRegistry`.

Though we use mallinfo() to track native allocation, this call itself can be expensive, and thus
we perform this check fairly rarely. More precisely, we do so only after the application has
called `NativeAllocationRegistry.registerNativeAllocation()` a certain number of times or
with a sufficiently large argument, or after `VMRuntime.registerNativeAllocation` is called.
Thus an application not using these APIs, e.g. because it is running almost entirely native
code, may never do so. This can be useful, in that an application that runs basically only
native code, and thus deallocates its own native memory, does not trigger the GC.

The actual computation for triggering a native-allocation-GC is performed by
`Heap::NativeMemoryOverTarget()`. This computes and compares two quantities:

1. An adjusted heap size for GC triggering. This consists of the Java heap size at which we would
   normally trigger a GC plus an allowance for native heap size. This allowance currently consists
   of one half (background processes) or three halves (foreground processes) of
   `NativeAllocationGcWatermark()`. The latter is `HeapMaxFree` (typically 32MB) plus 1/8 of the
   currently targeted heap size. For a foreground process, this allowance would typically be in
   the 50-100 MB range for something other than a low-end device.

2. An adjusted count of current bytes allocated. This is basically the number of bytes currently
   allocated in the Java heap, plus half the net number of native bytes allocated since the last
   GC. The latter is computed as the change since the last GC in the total-bytes-allocated
   obtained from `mallinfo()` plus `native_bytes_registered_`. The `native_bytes_registered_`
   field tracks the bytes explicitly registered, and not yet unregistered via
   `registerNativeAllocation` APIs. It excludes bytes registered as malloc-allocated via
   `NativeAllocationRegistry`. (The computation also considers the total amount of native memory
   currently allocated by this metric, as opposed to the change since the last GC. But that is
   currently de-weighted to the point of insignificance.)

A background GC is triggered if the second quantity exceeds the first. A stop-the-world-GC is
triggered if the second quantity is at least 4 times larger than the first, and the native heap
currently occupies a large fraction of device memory, suggesting that the GC is falling behind and
endangering device usability.

The fact that we consider both Java and native memory use at once means that we are more likely to
trigger a native GC when we are closer to the normal Java GC threshold.

The actual use of `mallinfo()` to compute native heap occupancy reflects experiments with
different `libc` implementations. These have different performance characteristics, and sometimes
disagree on the interpretation of `struct mallinfo` fields. We believe our current implementation
is solid with scudo and jemalloc on Android, and minimally usable for testing elsewhere.

(Some of this assumes typical current (May 2024) configuration constants, and may need to be
updated.)

Workaround for excessive native GCs
-----------------------------------

If an application routinely allocates and deallocates large amounts of memory without requiring
the GC, it may be preferable to bypass the system allocator, for example by using mmap directly.
This will avoid unnecessary GC triggering. This is clearly much more convenient for a small number
of large allocations than for small allocations. It could also be addressed with a new ART API,
but so far we have not found this necessary.