Docs: Add Debugging ART garbage collection

Bug: 22562924
Change-Id: Ib8ea621b1cf02a056f19c36f4ffd455c985abc85
diff --git a/src/devices/devices_toc.cs b/src/devices/devices_toc.cs
index 82a4750..09c6396 100644
--- a/src/devices/devices_toc.cs
+++ b/src/devices/devices_toc.cs
@@ -182,6 +182,7 @@
           <li><a href="<?cs var:toroot ?>devices/tech/dalvik/instruction-formats.html">Instruction Formats</a></li>
           <li><a href="<?cs var:toroot ?>devices/tech/dalvik/constraints.html">Constraints</a></li>
           <li><a href="<?cs var:toroot ?>devices/tech/dalvik/configure.html">Configuration</a></li>
+          <li><a href="<?cs var:toroot ?>devices/tech/dalvik/gc-debug.html">Garbage Collection</a></li>
         </ul>
       </li>
 
diff --git a/src/devices/tech/dalvik/gc-debug.jd b/src/devices/tech/dalvik/gc-debug.jd
new file mode 100644
index 0000000..676d38b
--- /dev/null
+++ b/src/devices/tech/dalvik/gc-debug.jd
@@ -0,0 +1,449 @@
+page.title=Debugging ART Garbage Collection
+@jd:body
+
+<!--
+    Copyright 2015 The Android Open Source Project
+
+    Licensed under the Apache License, Version 2.0 (the "License");
+    you may not use this file except in compliance with the License.
+    You may obtain a copy of the License at
+
+        http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing, software
+    distributed under the License is distributed on an "AS IS" BASIS,
+    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+    See the License for the specific language governing permissions and
+    limitations under the License.
+-->
+
+
+<div id="qv-wrapper">
+<div id="qv">
+  <h2 id="Contents">In this document</h2>
+  <ol id="auto-toc">
+  </ol>
+</div>
+</div>
+
+<p>This document describes how to debug Android Runtime (ART) Garbage Collection
+(GC) correctness and performance issues. It explains how to use GC verification
+options, identify solutions for GC verification failures, and measure and
+address GC performance problems.</p>
+
+<p>See <a href="index.html">ART and Dalvik</a>, the <a
+href="dex-format.html">Dalvik Executable format</a>, and the remaining pages
+within this <a href="index.html">ART and Dalvik</a> section to work with
+ART. See <a
+href="http://developer.android.com/guide/practices/verifying-apps-art.html">Verifying
+App Behavior on the Android Runtime (ART)</a> for additional help verifying app
+behavior.</p>
+
+<h2 id=art_gc_overview>ART GC overview</h2>
+
+<p>ART, introduced as a developer option in Android 4.4, is the default Android
+runtime for Android 5.0 and beyond. The Dalvik runtime is no longer maintained
+or available and its byte-code format is now used by ART.  Please note this
+section merely summarizes ART’s GC. For additional information, see the <a
+href="https://www.google.com/events/io/io14videos/b750c8da-aebe-e311-b297-00155d5066d7">Android
+runtime</a> presentation conducted at Google I/O 2014. </p>
+
+<p>ART has a few different GC plans that consist of running different garbage
+collectors. The default plan is the CMS (concurrent mark sweep) plan, which
+uses mostly sticky CMS and partial CMS. Sticky CMS is ART’s non-moving
+generational garbage collector. It scans only the portion of the heap that was
+modified since the last GC and can reclaim only the objects allocated since the
+last GC. In addition to the CMS plan, ART performs heap compaction when an app
+changes process state to a jank-imperceptible process state (e.g. background or
+cached).</p>
+
+<p>Aside from the new garbage collectors, ART also introduces a new bitmap-based
+memory allocator called RosAlloc (runs of slots allocator). This new allocator
+has sharded locking and outperforms DlMalloc by adding thread local buffers for
+small allocation sizes.</p>
+
+<p>Compared to Dalvik, the ART CMS garbage collection plan has a number of
+improvements:</p>
+
+<ul>
+  <li>The number of pauses is reduced from two to one compared to Dalvik.
+Dalvik’s first pause, which did mostly root marking, is done concurrently in
+ART by getting the threads to mark their own roots, then resume running right away.
+  <li>Similarly to Dalvik, the ART GC also has a pause before the sweeping process.
+The key difference in this area is that some of the Dalvik phases during this
+pause are done concurrently in ART. These phases include
+<code>java.lang.ref.Reference</code> processing, system weak sweeping (e.g. jni
+weak globals, etc.), re-marking non-thread roots, and card pre-cleaning. The
+phases that are still done paused in ART are scanning the dirty cards and
+re-marking the thread roots, which helps reduce the pause time.
+  <li>The last area where the ART GC improves over Dalvik is with increased GC
+throughput enabled by the sticky CMS collector. Unlike normal generational GC,
+sticky CMS is non-moving. Instead of having a dedicated region for young
+objects, young objects are kept in an allocation stack, which is basically an
+array of <code>java.lang.Object</code>. This avoids moving objects required to
+maintain low pauses but has the disadvantage of having longer collections for
+heaps with complex object graphs.
+</ul>
+
+<p>The other main other area where the ART GC is different than Dalvik is the
+introduction of moving garbage collectors. The goal of moving GCs is to
+reduce memory usage of backgrounded apps through heap compaction. Currently,
+the event that triggers heap compaction is ActivityManager process-state
+changes. When an app goes to background, it notifies ART the process state is
+no longer jank “perceptible.” This enables ART do things that cause long
+application thread pauses, such as compaction and monitor deflation. The two
+current moving GCs that are in use are homogeneous space compaction and
+semi-space compaction.</p>
+
+<ul>
+  <li>Semi-space compaction moves objects between two tightly packed bump pointer
+spaces. This moving GC occurs on low-memory devices since it saves slightly
+more memory than homogeneous space compaction. The additional savings come
+mostly from having tightly packed objects, which avoid RosAlloc / DlMalloc
+allocator accounting overhead. Since CMS is still used in the foreground and it
+can’t collect from a bump pointer space, semi space requires another transition
+when the app is foregrounded. This is not ideal since it can cause a noticeable pause.
+  <li>Homogenous space compaction works by copying from one RosAlloc space to another
+RosAlloc space. This helps reduce memory usage by reducing heap fragmentation.
+This is currently the default compaction mode for non-low-memory devices. The
+main advantage that homogeneous space compaction has over semi-space compaction
+is not needing a heap transition when the app goes back to foreground.
+</ul>
+
+<h2 id=gc_verification_and_performance_options>GC verification and performance options</h2>
+
+<p>It is possible to change the GC type if you are an OEM. The process for doing
+this involves modifying system properties through adb. Keep in mind that these
+are only modifiable on non-user or rooted builds.</p>
+
+<h3 id=changing_the_gc_type>Changing the GC type</h3>
+
+<p>There are ways to change the GC plans that ART uses. The main way to change the
+foreground GC plan is by changing the <code>dalvik.vm.gctype</code> property or
+passing in an <code>-Xgc:</code> option. It is possible to pass in multiple GC
+options separated by commas.</p>
+
+<p>In order to derive the entire list of available <code>-Xgc</code> settings,
+it is possible to type <code>adb shell dalvikvm -help</code> to print the
+various runtime command-line options.</p>
+
+<p>Here is one example that changes the GC to semi space and turns on pre-GC heap
+verification:
+<code>adb shell setprop dalvik.vm.gctype SS,preverify</code></p>
+
+<ul>
+  <li><code>CMS</code>, which is also the default value, specifies the
+concurrent mark sweep GC plan.  This plan consists of running sticky
+generational CMS, partial CMS, and full CMS. The allocator for this plan is
+RosAlloc for movable objects and DlMalloc for non-movable objects.
+  <li><code>SS</code> specifies the semi space GC plan. This plan has two semi
+spaces for movable objects and a DlMalloc space for non-movable objects. The
+movable object allocator defaults to a shared bump pointer allocator which uses
+atomic operations. However, if the <code>-XX:UseTLAB</code> flag is also passed
+in, the allocator uses thread local bump pointer allocation.
+  <li><code>GSS</code> specifies the generational semi space plan. This plan is
+very similar to the semi-space plan with the exception that older-lived objects
+are promoted into a large RosAlloc space. This has the advantage of needing to
+copy significantly fewer objects for typical use cases.
+</ul>
+
+<h3 id=verifying_the_heap>Verifying the heap</h3>
+
+<p>Heap verification is probably the most useful GC option for debugging
+GC-related errors or heap corruption. Enabling heap verification causes the GC
+to check the correctness of the heap at a few points during the garbage
+collection process. Heap verification shares the same options as the ones that
+change the GC type. If enabled, heap verification verifies the roots and
+ensures that reachable objects reference only other reachable objects. GC
+verification is enabled by passing in the following -<code>Xgc</code> values:</p>
+
+<ul>
+  <li>If enabled, <code>[no]preverify</code> performs heap verification before starting the GC.
+  <li>If enabled, <code>[no]presweepingverify</code> performs heap verification
+before starting the garbage collector sweeping
+process.
+  <li>If enabled, <code>[no]postverify</code> performs heap verification after
+the GC has finished sweeping.
+  <li><code>[no]preverify_rosalloc</code>,
+<code>[no]postsweepingverify_rosalloc</code>,
+<code>[no]postverify_rosalloc</code> are also additional GC options that verify
+only the state of RosAlloc’s internal accounting. The main things verified are
+that magic values match expected constants, and free blocks of memory are all
+registered in the <code>free_page_runs_</code> map.
+</ul>
+
+<h3 id=using_the_tlab_allocator_option>Using the TLAB allocator option</h3>
+
+<p>Currently, the only option that changes the allocator used without affecting
+the active GC type is the TLAB option. This option is not available through
+system properties but can be enabled by passing in -<code>XX:UseTLAB</code> to
+<code>dalvikvm</code>. This option enables faster allocation by having a
+shorter allocation code path. Since this option requires using either the SS or
+GSS GC types, which have rather long pauses, it is not enabled by default.</p>
+
+<h2 id=performance>Performance</h2>
+
+<p>There are two main tools that can be used to measure GC performance. GC timing
+dumps and systrace. The most visual way to measure GC performance problems
+would be to use systrace to determine which GCs are causing long pauses or
+preempting application threads. Although the ART GC is relatively efficient,
+there are still a few ways to get performance problems through excessive
+allocations or bad mutator behavior.</p>
+
+<h3 id=ergonomics>Ergonomics</h3>
+
+<p>Compared to Dalvik, ART has a few key differences regarding GC ergonomics. One
+of the major improvements compared to Dalvik is no longer having GC for alloc
+in cases where we start the concurrent GC later than needed. However, there is
+one downside to this, not blocking on the GC can cause the heap to grow more
+than Dalvik in some circumstances. Fortunately, ART has heap compaction, which
+mitigates this issue by defragmenting the heap when the process changes to a
+background process state.</p>
+
+<p>The CMS GC ergonomics have two types of GCs that are regularly run. Ideally,
+the GC ergonomics will run the generational sticky CMS more often than the
+partial CMS. The GC does sticky CMS until the throughput (calculated by bytes
+freed / second of GC duration) of the last GC is less than the mean throughput
+of partial CMS. When this occurs, the ergonomics plan the next concurrent GC to
+be a partial CMS instead of sticky CMS. After the partial CMS completes, the
+ergonomics changes the next GC back to sticky CMS. One key factor that makes
+the ergonomics work is that sticky CMS doesn’t adjust the heap footprint limit
+after it completes. This causes sticky CMS to happen more and more often until
+the throughput is lower than partial CMS, which ends up growing the heap.</p>
+
+<h3 id=using_sigquit_to_obtain_gc_performance_info>Using SIGQUIT to obtain GC performance info</h3>
+
+<p>It is possible to get GC performance timings for apps by sending SIGQUIT to
+already running apps or passing in -<code>XX:DumpGCPerformanceOnShutdown</code>
+to <code>dalvikvm</code> when starting a command line program. When an app gets
+the ANR request signal (SIGQUIT) it dumps information related to its locks,
+thread stacks, and GC performance.</p>
+
+<p>The way to get GC timing dumps is to use:<br>
+<code>$ adb shell kill -S QUIT <pid></code></p>
+
+<p>This creates a <code>traces.txt</code> file in <code>/data/anr/</code>. This
+file contains some ANR dumps as well as GC timings. You can locate the
+GC timings by searching for: “Dumping cumulative Gc timings” These timings will
+show a few things that may be of interest. It will show the histogram info for
+each GC type’s phases and pauses. The pauses are usually more important to look
+at. For example:</p>
+
+<pre>
+sticky concurrent mark sweep paused:	Sum: 5.491ms 99% C.I. 1.464ms-2.133ms Avg: 1.830ms Max: 2.133ms
+</pre>
+
+<p><code>This</code> shows that the average pause was 1.83ms. This should be low enough to not
+cause missed frames in most applications and shouldn’t be a concern.</p>
+
+<p>Another area of interest is time to suspend. What time to suspend measures is
+how long it takes a thread to reach a suspend point after the GC requests that
+it suspends. This time is included in the GC pauses, so it is useful to
+determine if long pauses are caused by the GC being slow or the thread
+suspending slowly. Here is an example of what a normal time to suspend
+resembles on a Nexus 5:</p>
+
+<pre>
+suspend all histogram:	Sum: 1.513ms 99% C.I. 3us-546.560us Avg: 47.281us Max: 601us
+</pre>
+
+<p>There are also a few other areas of interest, such as total time spent, GC
+throughput, etc. Examples:</p>
+
+<pre>
+Total time spent in GC: 502.251ms
+Mean GC size throughput: 92MB/s
+Mean GC object throughput: 1.54702e+06 objects/s
+</pre>
+
+<p>Here is an example of how to dump the GC timings of an already running app:
+
+<pre>
+$ adb shell kill -s QUIT &lt;pid&gt;
+$ adb pull /data/anr/traces.txt
+</pre>
+
+<p>At this point the GC timings are inside of traces.txt. Here is example output
+from Google maps:</p>
+
+<pre>
+Start Dumping histograms for 34 iterations for sticky concurrent mark sweep
+ScanGrayAllocSpaceObjects:	Sum: 196.174ms 99% C.I. 0.011ms-11.615ms Avg: 1.442ms Max: 14.091ms
+FreeList:	Sum: 140.457ms 99% C.I. 6us-1676.749us Avg: 128.505us Max: 9886us
+MarkRootsCheckpoint:	Sum: 110.687ms 99% C.I. 0.056ms-9.515ms Avg: 1.627ms Max: 10.280ms
+SweepArray:	Sum: 78.727ms 99% C.I. 0.121ms-11.780ms Avg: 2.315ms Max: 12.744ms
+ProcessMarkStack:	Sum: 77.825ms 99% C.I. 1.323us-9120us Avg: 576.481us Max: 10185us
+(Paused)ScanGrayObjects:	Sum: 32.538ms 99% C.I. 286us-3235.500us Avg: 986us Max: 3434us
+AllocSpaceClearCards:	Sum: 30.592ms 99% C.I. 10us-2249.999us Avg: 224.941us Max: 4765us
+MarkConcurrentRoots:	Sum: 30.245ms 99% C.I. 3us-3017.999us Avg: 444.779us Max: 3774us
+ReMarkRoots:	Sum: 13.144ms 99% C.I. 66us-712us Avg: 386.588us Max: 712us
+ScanGrayImageSpaceObjects:	Sum: 13.075ms 99% C.I. 29us-2538.999us Avg: 192.279us Max: 3080us
+MarkingPhase:	Sum: 9.743ms 99% C.I. 170us-518us Avg: 286.558us Max: 518us
+SweepSystemWeaks:	Sum: 8.046ms 99% C.I. 28us-479us Avg: 236.647us Max: 479us
+MarkNonThreadRoots:	Sum: 5.215ms 99% C.I. 31us-698.999us Avg: 76.691us Max: 703us
+ImageModUnionClearCards:	Sum: 2.708ms 99% C.I. 26us-92us Avg: 39.823us Max: 92us
+ScanGrayZygoteSpaceObjects:	Sum: 2.488ms 99% C.I. 19us-250.499us Avg: 37.696us Max: 295us
+ResetStack:	Sum: 2.226ms 99% C.I. 24us-449us Avg: 65.470us Max: 452us
+ZygoteModUnionClearCards:	Sum: 2.124ms 99% C.I. 18us-233.999us Avg: 32.181us Max: 291us
+FinishPhase:	Sum: 1.881ms 99% C.I. 31us-431.999us Avg: 55.323us Max: 466us
+RevokeAllThreadLocalAllocationStacks:	Sum: 1.749ms 99% C.I. 8us-349us Avg: 51.441us Max: 377us
+EnqueueFinalizerReferences:	Sum: 1.513ms 99% C.I. 3us-201us Avg: 44.500us Max: 201us
+ProcessReferences:	Sum: 438us 99% C.I. 3us-212us Avg: 12.882us Max: 212us
+ProcessCards:	Sum: 381us 99% C.I. 4us-17us Avg: 5.602us Max: 17us
+PreCleanCards:	Sum: 363us 99% C.I. 8us-17us Avg: 10.676us Max: 17us
+ReclaimPhase:	Sum: 357us 99% C.I. 7us-91.500us Avg: 10.500us Max: 93us
+(Paused)PausePhase:	Sum: 312us 99% C.I. 7us-15us Avg: 9.176us Max: 15us
+SwapBitmaps:	Sum: 166us 99% C.I. 4us-8us Avg: 4.882us Max: 8us
+(Paused)ScanGrayAllocSpaceObjects:	Sum: 126us 99% C.I. 14us-112us Avg: 63us Max: 112us
+MarkRoots:	Sum: 121us 99% C.I. 2us-7us Avg: 3.558us Max: 7us
+(Paused)ScanGrayImageSpaceObjects:	Sum: 68us 99% C.I. 68us-68us Avg: 68us Max: 68us
+BindBitmaps:	Sum: 50us 99% C.I. 1us-3us Avg: 1.470us Max: 3us
+UnBindBitmaps:	Sum: 49us 99% C.I. 1us-3us Avg: 1.441us Max: 3us
+SwapStacks:	Sum: 47us 99% C.I. 1us-3us Avg: 1.382us Max: 3us
+RecordFree:	Sum: 42us 99% C.I. 1us-3us Avg: 1.235us Max: 3us
+ForwardSoftReferences:	Sum: 37us 99% C.I. 1us-2us Avg: 1.121us Max: 2us
+InitializePhase:	Sum: 36us 99% C.I. 1us-2us Avg: 1.058us Max: 2us
+FindDefaultSpaceBitmap:	Sum: 32us 99% C.I. 250ns-1000ns Avg: 941ns Max: 1000ns
+(Paused)ProcessMarkStack:	Sum: 5us 99% C.I. 250ns-3000ns Avg: 147ns Max: 3000ns
+PreSweepingGcVerification:	Sum: 0 99% C.I. 0ns-0ns Avg: 0ns Max: 0ns
+Done Dumping histograms
+sticky concurrent mark sweep paused:	Sum: 63.268ms 99% C.I. 0.308ms-8.405ms
+Avg: 1.860ms Max: 8.883ms
+sticky concurrent mark sweep total time: 763.787ms mean time: 22.464ms
+sticky concurrent mark sweep freed: 1072342 objects with total size 75MB
+sticky concurrent mark sweep throughput: 1.40543e+06/s / 98MB/s
+Total time spent in GC: 4.805s
+Mean GC size throughput: 18MB/s
+Mean GC object throughput: 330899 objects/s
+Total number of allocations 2015049
+Total bytes allocated 177MB
+Free memory 4MB
+Free memory until GC 4MB
+Free memory until OOME 425MB
+Total memory 90MB
+Max memory 512MB
+Zygote space size 4MB
+Total mutator paused time: 229.566ms
+Total time waiting for GC to complete: 187.655us
+</pre>
+
+<h2 id=tools_for_analyzing_gc_correctness_problems>Tools for analyzing GC correctness problems</h2>
+
+<p>There are various things that can cause crashes inside of ART. Crashes that
+occur reading or writing to object fields may indicate heap corruption. If the
+GC crashes when it is running, it could also point to heap corruption. There
+are various things that can cause heap corruption, the most common cause is
+probably incorrect app code. Fortunately, there are tools to debug GC and
+heap-related crashes. These include the heap verification options specified
+above, valgrind, and CheckJNI.</p>
+
+<h3 id=checkjni>CheckJNI</h3>
+
+<p>Another way to verify app behavior is to use CheckJNI. CheckJNI is a mode that
+adds additional JNI checks; these aren’t enabled by default due to performance
+reasons. The checks will catch a few errors that could cause heap corruption
+such as using invalid/stale local and global references. Here is how to enable
+CheckJNI:</p>
+
+<pre>
+$ adb shell setprop dalvik.vm.checkjni true
+</pre>
+
+<p>Forcecopy mode is another part of CheckJNI that is very useful for detecting
+writes past the end of array regions. When enabled, forcecopy causes the array
+access JNI functions to always return copies with red zones. A <em>red
+zone</em> is a region at the end/start of the returned pointer that has a
+special value, which is verified when the array is released. If the values in
+the red zone don’t match what is expected, this usually means a buffer overrun
+or underrun occurred. This would cause CheckJNI to abort. Here is how to enable
+forcecopy mode:</p>
+
+<pre>
+$ adb shell setprop dalvik.vm.jniopts forcecopy
+</pre>
+
+<p>One example of an error that CheckJNI should catch is writing past the end of
+an array obtained from <code>GetPrimitiveArrayCritical</code>. This operation
+would very likely corrupt the Java heap. If the write is
+within the CheckJNI red zone area, then CheckJNI would catch the issue when the
+corresponding <code>ReleasePrimitiveArrayCritical</code> is called. Otherwise,
+the write will end up corrupting some random object in
+the Java heap and possibly causing a future GC crash. If the corrupted memory
+is a reference field, then the GC may catch the error and print a “<em>Tried to
+mark <ptr> not contained by any spaces</em>” error.</p>
+
+<p>This error occurs when the GC attempts to mark an object for which it can’t
+find a space. After this check fails, the GC traverses the roots and tries to
+see if the invalid object is a root. From here, there are two options: The
+object is a root or a non-root object.</p>
+
+<h3 id=valgrind>Valgrind</h3>
+
+<p>The ART heap supports optional valgrind instrumentation, which provides a
+way to detect reads and writes to and from an invalid heap address. ART detects
+when the app is running under valgrind and inserts red zones before and after
+each object allocation. If there are any reads or writes to these red zones,
+valgrind prints an error. One example of when this could happen is if you read
+or write past the end of an array’s elements while using direct array access
+through JNI. Since the AOT compilers use implicit null checks, it is
+recommended to use eng builds for running valgrind. Another thing to note is
+that valgrind is orders of magnitude slower than normal execution.</p>
+
+<p>Here is an example use:</p>
+
+<pre>
+# build and install
+$ mmm external/valgrind
+$ adb remount && adb sync
+# disable selinux
+$ adb shell setenforce 0
+$ adb shell setprop wrap.com.android.calculator2
+"TMPDIR=/data/data/com.android.calculator2 logwrapper valgrind"
+# push symbols
+$ adb shell mkdir /data/local/symbols
+$ adb push $OUT/symbols /data/local/symbols
+$ adb logcat
+</pre>
+
+
+<h3 id=invalid_root_example>Invalid root example</h3>
+
+<p>In the case where the object is actually an invalid root, it will print some
+useful information:
+<code>art E  5955  5955 art/runtime/gc/collector/mark_sweep.cc:383] Tried to mark 0x2
+not contained by any spaces</code></p>
+
+<pre>
+art E  5955  5955 art/runtime/gc/collector/mark_sweep.cc:384] Attempting see if
+it's a bad root
+art E  5955  5955 art/runtime/gc/collector/mark_sweep.cc:485] Found invalid
+root: 0x2
+art E  5955  5955 art/runtime/gc/collector/mark_sweep.cc:486]
+Type=RootJavaFrame thread_id=1 location=Visiting method 'java.lang.Object
+com.google.gwt.corp.collections.JavaReadableJsArray.get(int)' at dex PC 0x0002
+(native PC 0xf19609d9) vreg=1
+</pre>
+
+<p>In this case, <code>vreg 1</code> inside of
+<code>com.google.gwt.corp.collections.JavaReadableJsArray.get</code> is
+supposed to contain a heap reference but actually contains an invalid pointer
+of address <code>0x2</code>. This is clearly an invalid root. The next step to
+debug this issue would be to use <code>oatdump</code> on the oat file and look
+at the method that has the invalid root. In this case, the error turned out to
+be a compiler bug in the x86 backend. Here is the changelist that fixed it: <a
+href="https://android-review.googlesource.com/#/c/133932/">https://android-review.googlesource.com/#/c/133932/</a></p>
+
+<h3 id=corrupted_object_example>Corrupted object example</h3>
+
+<p>In the case where the object isn’t a root, output similar to the following
+prints:</p>
+
+<pre>
+01-15 12:38:00.196  1217  1238 E art     : Attempting see if it's a bad root
+01-15 12:38:00.196  1217  1238 F art     :
+art/runtime/gc/collector/mark_sweep.cc:381] Can't mark invalid object
+</pre>
+
+<p>When heap corruption isn’t an invalid root, it is unfortunately hard to debug.
+This error message indicates that there was at least one object in the heap
+that was pointing to the invalid object.</p>