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Debugging ART Garbage Collection

This page 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.

To work with ART, see the pages in this ART and Dalvik section, and the Dalvik Executable format. For additional help verifying app behavior, see Verifying app behavior on the Android runtime (ART).

ART GC overview

ART has a few different GC plans that consist of running different garbage collectors. Starting with Android 8 (Oreo), the default plan is Concurrent Copying (CC). The other GC plan is Concurrent Mark Sweep (CMS).

Some of the main characterstics of Concurrent Copying GC are:

  • CC enables use of a bump-pointer allocator called RegionTLAB. This allocates a thread-local allocation buffer (TLAB) to each app thread, which can then allocate objects out of its TLAB by bumping the "top" pointer, without any synchronization.
  • CC performs heap defragmentation by concurrently copying objects without pausing app threads. This is achieved with the help of a read-barrier which intercepts reference reads from the heap, without the need of any intervention from the app developer.
  • GC only has one small pause, which is constant in time with regards to the heap size.
  • CC extends to be a generational GC in Android 10 and higher. It enables collecting young objects, which often become unreachable fairly quickly, with little effort. This helps by increasing GC throughput and considerably delaying the need to perform a full-heap GC.

The other GC that ART still supports is CMS. This GC also supports compaction, but not concurrently. Compaction is avoided until the app goes into the background, at which time the app threads are paused to perform compaction. Compaction also becomes necessary when an object allocation fails due to fragmentation. In this case the app may potentially become unresponsive for some time.

Since CMS rarely compacts, and thus free objects may not be contiguous, it uses a free-list-based allocator called RosAlloc. It has a higher allocation cost as compared to RegionTLAB. Finally, because of internal fragmentation the memory usage for the Java heap can be higher for CMS than CC.

GC verification and performance options

Changing the GC type

OEMs can change the GC type. The change process involves setting the ART_USE_READ_BARRIER environment variable at build time. The default value is true, which enables the CC collector as it uses the read-barrier. For CMS this variable should be explicitly set to false.

By default, the CC collector runs in generational mode in Android 10 and higher. To disable generational mode, the -Xgc:nogenerational_cc command-line argument can be used. Alternatively, the system property can be set as follows:

adb shell setprop dalvik.vm.gctype nogenerational_cc
The CMS collector always runs in generational mode.

Verifying the heap

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 -Xgc values:

  • If enabled, [no]preverify performs heap verification before starting the GC.
  • If enabled, [no]presweepingverify performs heap verification before starting the garbage collector sweeping process.
  • If enabled, [no]postverify performs heap verification after the GC finishes sweeping.
  • [no]preverify_rosalloc, [no]postsweepingverify_rosalloc, and [no]postverify_rosalloc are additional GC options that verify only the state of RosAlloc’s internal accounting. Therefore, they are only applicable with the CMS collector, which uses RosAlloc allocator. The main things verified are that magic values match expected constants, and free blocks of memory are all registered in the free_page_runs_ map.

Performance

There are two main tools to measure GC performance, GC timing dumps and Systrace. There is also an advanced version of Systrace, called Perfetto. The visual way to measure GC performance problems is to use Systrace and Perfetto to determine which GCs are causing long pauses or preempting app threads. Although the ART GC has significantly improved over time, bad mutator behavior, such as excessive allocation, can still cause performance problems

Collection strategy

The CC GC collects by either running a young GC or a full-heap GC. Ideally the young GC is run more often. The GC does young CC collections until the throughput (calculated by bytes freed/second of GC duration) of the just finished collection cycle is less than the mean throughput of full-heap CC collections. When this occurs, the full-heap CC is chosen for the next concurrent GC instead of young CC. After the full-heap collection completes, the next GC is switched back to young CC. One key factor that makes this strategy work is that young CC doesn’t adjust the heap footprint limit after it completes. This causes young CC to happen more and more often until the throughput is lower than full-heap CC, which ends up growing the heap.

Using SIGQUIT to obtain GC performance info

To get GC performance timings for apps, send SIGQUIT to already running apps or pass in -XX:DumpGCPerformanceOnShutdown to dalvikvm 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.

To get GC timing dumps, use:

adb shell kill -S QUIT PID

This creates a file (with the date and time in the name such as anr_2020-07-13-19-23-39-817) in /data/anr/. 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 show a few things that may be of interest, including the histogram info for each GC type’s phases and pauses. The pauses are usually more important to look at. For example:

young concurrent copying paused:	Sum: 5.491ms 99% C.I. 1.464ms-2.133ms Avg: 1.830ms Max: 2.133ms

This shows that the average pause was 1.83 ms, which should be low enough that it doesn't cause missed frames in most apps and shouldn’t be a concern.

Another area of interest is time to suspend, which measures 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's useful to determine if long pauses are caused by the GC being slow or the thread suspending slowly. Here's an example of a normal time to suspend on a Nexus 5:

suspend all histogram:	Sum: 1.513ms 99% C.I. 3us-546.560us Avg: 47.281us Max: 601us

There are other areas of interest, including total time spent and GC throughput. Examples:

Total time spent in GC: 502.251ms
Mean GC size throughput: 92MB/s
Mean GC object throughput: 1.54702e+06 objects/s

Here's an example of how to dump the GC timings of an already running app:

adb shell kill -s QUIT PID
adb pull /data/anr/anr_2020-07-13-19-23-39-817

At this point the GC timings are inside of anr_2020-07-13-19-23-39-817. Here's example output from Google Maps:

Start Dumping histograms for 2195 iterations for concurrent copying
MarkingPhase:   Sum: 258.127s 99% C.I. 58.854ms-352.575ms Avg: 117.651ms Max: 641.940ms
ScanCardsForSpace:      Sum: 85.966s 99% C.I. 15.121ms-112.080ms Avg: 39.164ms Max: 662.555ms
ScanImmuneSpaces:       Sum: 79.066s 99% C.I. 7.614ms-57.658ms Avg: 18.014ms Max: 546.276ms
ProcessMarkStack:       Sum: 49.308s 99% C.I. 6.439ms-81.640ms Avg: 22.464ms Max: 638.448ms
ClearFromSpace: Sum: 35.068s 99% C.I. 6.522ms-40.040ms Avg: 15.976ms Max: 633.665ms
SweepSystemWeaks:       Sum: 14.209s 99% C.I. 3.224ms-15.210ms Avg: 6.473ms Max: 201.738ms
CaptureThreadRootsForMarking:   Sum: 11.067s 99% C.I. 0.835ms-13.902ms Avg: 5.044ms Max: 25.565ms
VisitConcurrentRoots:   Sum: 8.588s 99% C.I. 1.260ms-8.547ms Avg: 1.956ms Max: 231.593ms
ProcessReferences:      Sum: 7.868s 99% C.I. 0.002ms-8.336ms Avg: 1.792ms Max: 17.376ms
EnqueueFinalizerReferences:     Sum: 3.976s 99% C.I. 0.691ms-8.005ms Avg: 1.811ms Max: 16.540ms
GrayAllDirtyImmuneObjects:      Sum: 3.721s 99% C.I. 0.622ms-6.702ms Avg: 1.695ms Max: 14.893ms
SweepLargeObjects:      Sum: 3.202s 99% C.I. 0.032ms-6.388ms Avg: 1.458ms Max: 549.851ms
FlipOtherThreads:       Sum: 2.265s 99% C.I. 0.487ms-3.702ms Avg: 1.031ms Max: 6.327ms
VisitNonThreadRoots:    Sum: 1.883s 99% C.I. 45us-3207.333us Avg: 429.210us Max: 27524us
InitializePhase:        Sum: 1.624s 99% C.I. 231.171us-2751.250us Avg: 740.220us Max: 6961us
ForwardSoftReferences:  Sum: 1.071s 99% C.I. 215.113us-2175.625us Avg: 488.362us Max: 7441us
ReclaimPhase:   Sum: 490.854ms 99% C.I. 32.029us-6373.807us Avg: 223.623us Max: 362851us
EmptyRBMarkBitStack:    Sum: 479.736ms 99% C.I. 11us-3202.500us Avg: 218.558us Max: 13652us
CopyingPhase:   Sum: 399.163ms 99% C.I. 24us-4602.500us Avg: 181.851us Max: 22865us
ThreadListFlip: Sum: 295.609ms 99% C.I. 15us-2134.999us Avg: 134.673us Max: 13578us
ResumeRunnableThreads:  Sum: 238.329ms 99% C.I. 5us-2351.250us Avg: 108.578us Max: 10539us
ResumeOtherThreads:     Sum: 207.915ms 99% C.I. 1.072us-3602.499us Avg: 94.722us Max: 14179us
RecordFree:     Sum: 188.009ms 99% C.I. 64us-312.812us Avg: 85.653us Max: 2709us
MarkZygoteLargeObjects: Sum: 133.301ms 99% C.I. 12us-734.999us Avg: 60.729us Max: 10169us
MarkStackAsLive:        Sum: 127.554ms 99% C.I. 13us-417.083us Avg: 58.111us Max: 1728us
FlipThreadRoots:        Sum: 126.119ms 99% C.I. 1.028us-3202.499us Avg: 57.457us Max: 11412us
SweepAllocSpace:        Sum: 117.761ms 99% C.I. 24us-400.624us Avg: 53.649us Max: 1541us
SwapBitmaps:    Sum: 56.301ms 99% C.I. 10us-125.312us Avg: 25.649us Max: 1475us
(Paused)GrayAllNewlyDirtyImmuneObjects: Sum: 33.047ms 99% C.I. 9us-49.931us Avg: 15.055us Max: 72us
(Paused)SetFromSpace:   Sum: 11.651ms 99% C.I. 2us-49.772us Avg: 5.307us Max: 71us
(Paused)FlipCallback:   Sum: 7.693ms 99% C.I. 2us-32us Avg: 3.504us Max: 32us
(Paused)ClearCards:     Sum: 6.371ms 99% C.I. 250ns-49753ns Avg: 207ns Max: 188000ns
Sweep:  Sum: 5.793ms 99% C.I. 1us-49.818us Avg: 2.639us Max: 93us
UnBindBitmaps:  Sum: 5.255ms 99% C.I. 1us-31us Avg: 2.394us Max: 31us
Done Dumping histograms
concurrent copying paused:      Sum: 315.249ms 99% C.I. 49us-1378.125us Avg: 143.621us Max: 7722us
concurrent copying freed-bytes: Avg: 34MB Max: 54MB Min: 2062KB
Freed-bytes histogram: 0:4,5120:5,10240:19,15360:69,20480:167,25600:364,30720:529,35840:405,40960:284,46080:311,51200:38
concurrent copying total time: 569.947s mean time: 259.657ms
concurrent copying freed: 1453160493 objects with total size 74GB
concurrent copying throughput: 2.54964e+06/s / 134MB/s  per cpu-time: 157655668/s / 150MB/s
Average major GC reclaim bytes ratio 0.486928 over 2195 GC cycles
Average major GC copied live bytes ratio 0.0894662 over 2199 major GCs
Cumulative bytes moved 6586367960
Cumulative objects moved 127490240
Peak regions allocated 376 (94MB) / 2048 (512MB)
Start Dumping histograms for 685 iterations for young concurrent copying
ScanCardsForSpace:      Sum: 26.288s 99% C.I. 8.617ms-77.759ms Avg: 38.377ms Max: 432.991ms
ProcessMarkStack:       Sum: 21.829s 99% C.I. 2.116ms-71.119ms Avg: 31.868ms Max: 98.679ms
ClearFromSpace: Sum: 19.420s 99% C.I. 5.480ms-50.293ms Avg: 28.351ms Max: 507.330ms
ScanImmuneSpaces:       Sum: 9.968s 99% C.I. 8.155ms-30.639ms Avg: 14.552ms Max: 46.676ms
SweepSystemWeaks:       Sum: 6.741s 99% C.I. 3.655ms-14.715ms Avg: 9.841ms Max: 22.142ms
GrayAllDirtyImmuneObjects:      Sum: 4.466s 99% C.I. 0.584ms-14.315ms Avg: 6.519ms Max: 24.355ms
FlipOtherThreads:       Sum: 3.672s 99% C.I. 0.631ms-16.630ms Avg: 5.361ms Max: 18.513ms
ProcessReferences:      Sum: 2.806s 99% C.I. 0.001ms-9.459ms Avg: 2.048ms Max: 11.951ms
EnqueueFinalizerReferences:     Sum: 1.857s 99% C.I. 0.424ms-8.609ms Avg: 2.711ms Max: 24.063ms
VisitConcurrentRoots:   Sum: 1.094s 99% C.I. 1.306ms-5.357ms Avg: 1.598ms Max: 6.831ms
SweepArray:     Sum: 711.032ms 99% C.I. 0.022ms-3.502ms Avg: 1.038ms Max: 7.307ms
InitializePhase:        Sum: 667.346ms 99% C.I. 303us-2643.749us Avg: 974.227us Max: 3199us
VisitNonThreadRoots:    Sum: 388.145ms 99% C.I. 103.911us-1385.833us Avg: 566.635us Max: 5374us
ThreadListFlip: Sum: 202.730ms 99% C.I. 18us-2414.999us Avg: 295.956us Max: 6780us
EmptyRBMarkBitStack:    Sum: 132.934ms 99% C.I. 8us-1757.499us Avg: 194.064us Max: 8495us
ResumeRunnableThreads:  Sum: 109.593ms 99% C.I. 6us-4719.999us Avg: 159.989us Max: 11106us
ResumeOtherThreads:     Sum: 86.733ms 99% C.I. 3us-4114.999us Avg: 126.617us Max: 19332us
ForwardSoftReferences:  Sum: 69.686ms 99% C.I. 14us-2014.999us Avg: 101.731us Max: 4723us
RecordFree:     Sum: 58.889ms 99% C.I. 0.500us-185.833us Avg: 42.984us Max: 769us
FlipThreadRoots:        Sum: 58.540ms 99% C.I. 1.034us-4314.999us Avg: 85.459us Max: 10224us
CopyingPhase:   Sum: 52.227ms 99% C.I. 26us-728.749us Avg: 76.243us Max: 2060us
ReclaimPhase:   Sum: 37.207ms 99% C.I. 7us-2322.499us Avg: 54.316us Max: 3826us
(Paused)GrayAllNewlyDirtyImmuneObjects: Sum: 23.859ms 99% C.I. 11us-98.917us Avg: 34.830us Max: 128us
FreeList:       Sum: 20.376ms 99% C.I. 2us-188.875us Avg: 29.573us Max: 998us
MarkZygoteLargeObjects: Sum: 18.970ms 99% C.I. 4us-115.749us Avg: 27.693us Max: 122us
(Paused)SetFromSpace:   Sum: 12.331ms 99% C.I. 3us-94.226us Avg: 18.001us Max: 109us
SwapBitmaps:    Sum: 11.761ms 99% C.I. 5us-49.968us Avg: 17.169us Max: 67us
ResetStack:     Sum: 4.317ms 99% C.I. 1us-64.374us Avg: 6.302us Max: 190us
UnBindBitmaps:  Sum: 3.803ms 99% C.I. 4us-49.822us Avg: 5.551us Max: 70us
(Paused)ClearCards:     Sum: 3.336ms 99% C.I. 250ns-7000ns Avg: 347ns Max: 7000ns
(Paused)FlipCallback:   Sum: 3.082ms 99% C.I. 1us-30us Avg: 4.499us Max: 30us
Done Dumping histograms
young concurrent copying paused:        Sum: 229.314ms 99% C.I. 37us-2287.499us Avg: 334.764us Max: 6850us
young concurrent copying freed-bytes: Avg: 44MB Max: 50MB Min: 9132KB
Freed-bytes histogram: 5120:1,15360:1,20480:6,25600:1,30720:1,35840:9,40960:235,46080:427,51200:4
young concurrent copying total time: 100.823s mean time: 147.187ms
young concurrent copying freed: 519927309 objects with total size 30GB
young concurrent copying throughput: 5.15683e+06/s / 304MB/s  per cpu-time: 333152554/s / 317MB/s
Average minor GC reclaim bytes ratio 0.52381 over 685 GC cycles
Average minor GC copied live bytes ratio 0.0512109 over 685 minor GCs
Cumulative bytes moved 1542000944
Cumulative objects moved 28393168
Peak regions allocated 376 (94MB) / 2048 (512MB)
Total time spent in GC: 670.771s
Mean GC size throughput: 159MB/s per cpu-time: 177MB/s
Mean GC object throughput: 2.94152e+06 objects/s
Total number of allocations 1974199562
Total bytes allocated 104GB
Total bytes freed 104GB
Free memory 10MB
Free memory until GC 10MB
Free memory until OOME 442MB
Total memory 80MB
Max memory 512MB
Zygote space size 2780KB
Total mutator paused time: 544.563ms
Total time waiting for GC to complete: 117.494ms
Total GC count: 2880
Total GC time: 670.771s
Total blocking GC count: 1
Total blocking GC time: 86.373ms
Histogram of GC count per 10000 ms: 0:259879,1:2828,2:24,3:1
Histogram of blocking GC count per 10000 ms: 0:262731,1:1
Native bytes total: 30599192 registered: 8947416
Total native bytes at last GC: 30344912

Tools for analyzing GC correctness problems

Various things 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's running, it could also point to heap corruption. The most common cause of heap corruption is incorrect app code. Fortunately, there are tools to debug GC and heap-related crashes, including the heap verification options specified above, and CheckJNI.

CheckJNI

CheckJNI is a mode that adds JNI checks to verify app behavior; these aren’t enabled by default for performance reasons. The checks catch a few errors that could cause heap corruption, such as using invalid/stale local and global references. To enable CheckJNI:

adb shell setprop dalvik.vm.checkjni true

CheckJNI's forcecopy mode is useful for detecting writes past the end of array regions. When enabled, forcecopy causes the array access JNI functions to return copies with red zones. A red zone 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's expected, a buffer overrun or underrun occurred. This causes CheckJNI to abort. To enable forcecopy mode:

adb shell setprop dalvik.vm.jniopts forcecopy

An example of an error that CheckJNI should catch is writing past the end of an array obtained from GetPrimitiveArrayCritical. This operation can corrupt the Java heap. If the write is within the CheckJNI red zone area, then CheckJNI catches the issue when the corresponding ReleasePrimitiveArrayCritical is called. Otherwise, the write corrupts some random object in the Java heap and can cause a future GC crash. If the corrupted memory is a reference field, then the GC may catch the error and print the error Tried to mark <ptr> not contained by any spaces.

This error occurs when the GC attempts to mark an object that it can’t find a space for. 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 nonroot object.

Invalid root example

In the case where the object is an invalid root, it prints some useful information: art E 5955 5955 art/runtime/gc/collector/mark_sweep.cc:383] Tried to mark 0x2 not contained by any spaces

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

In this case, vreg=1 inside of com.google.gwt.corp.collections.JavaReadableJsArray.get is supposed to contain a heap reference, but contains an invalid pointer of address 0x2. This is an invalid root. To debug this issue, use oatdump on the oat file and look at the method with the invalid root. In this case, the error turned out to be a compiler bug in the x86 backend. Here's the changelist that fixed it: https://android-review.googlesource.com/#/c/133932/

Corrupted object example

If the object isn’t a root, output similar to the following prints:

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

When heap corruption isn’t an invalid root, it's hard to debug. This error message indicates that there was at least one object in the heap that was pointing to the invalid object.