Develop Reference

Using OpenCL with Android JNI produces slow code due to some overhead

I implemented an algorithm on android using OpenCL and OpenMP. The OpenMP implementation runs about 10 times slower than the OpenCL one.
OpenMP: ~250 ms
OpenCL: ~25 ms
But overall, if I measure the time from the java android side, I get roughly the same time to call and get my values.
For example:
Java code:
// calls C implementation using JNI (Java Native Interface)
bool useOpenCL = true;
myFunction(bitmap, useOpenCL); // ~300 ms, timed with System.nanoTime() here, but omitted code for clarity
myFunction(bitmap, !useOpenCL); // ~300 ms, timed with System.nanoTime() here, but omitted code for clarity
C code:
JNIEXPORT void JNICALL Java_com_xxxxx_myFunctionNative(JNIEnv * env, jobject obj, jobject pBitmap, jboolean useOpenCL)
{
// same before, setting some variables
clock_t startTimer, stopTimer;
startTimer = clock();
if ((bool) useOpenCL) {
calculateUsingOpenCL(); // runs in ~25 ms, timed here, using clock()
}
else {
calculateUsingOpenMP(); // runs in ~250 ms
}
stopTimer = clock();
__android_log_print(ANDROID_LOG_VERBOSE, APPNAME, "Time in ms: %f\n", 1000.0f* (float)(stopTimer - startTimer) / (float)CLOCKS_PER_SEC);
// same from here on, e.g.: copying values to java side
}
The Java code, in both cases executes roughly in the same time, around 300 ms. To be more precise, elapsedTime is a bit more for OpenCL, that is OpenCL is slower on average.
Looking at the individual run-times of the OpenMP, and OpenCL implementations, OpenCL version should be much faster overall. But for some reason, there is an overhead that I cannot find.
I also compared OpenCL vs Normal native code (no OpenMP), I still got the same results, with roughly same runtime overall, even though the calculateUsingOpenCL ran at least 10 times faster.
Ideas:
Maybe the GPU (in OpenCL case) is less efficient in general, because it has less memory available. There are few variables that we need to preallocate, which are used every frame. So, we checked the time it takes for android to draw a bitmap in both cases (OpenMP, OpenCL). In the OpenCL case, sometimes drawing a bitmap took longer (3 times longer), but not by the amount that would equalize the overall run time of the program.
Does JNI use GPU to accelerate some calls, which could cause the OpenCL version to be slower?
EDIT:
Is it possible that Java Garbage collection is triggered by OpenCL, causing the big overhead?

It turns out, clock() is unreliable (on Android), so instead we used the following method to measure time. With this method, everything is ok.
int64_t getTimeNsec() {
struct timespec now;
clock_gettime(CLOCK_MONOTONIC, &now);
return (int64_t) now.tv_sec*1000000000LL + now.tv_nsec;
}
clock_t startTimer, stopTimer;
startTimer = getTimeNsec();
function_to_measure();
stopTimer = getTimeNsec();
__android_log_print(ANDROID_LOG_VERBOSE, APPNAME, "Runtime in milliseconds (ms): %f", (float)(stopTimer - startTimer) / 1000000.0f);
This was suggested here:
How to obtain computation time in NDK

Trying to cause a native leak

I'm trying to cause a leak using this native method from my app. I can see "Method returned." in my Logs but I don't seem to lose any RAM. I'm using(MemoryInfo.availMem / 1048576L) for tracking usage.
JNIEXPORT jstring JNICALL Java_com_app_native_Wrapper_causeLeak(JNIEnv *je, jclass jc, jint bytes) {
char *p_array = calloc(bytes,sizeof(char));
return (*je)->NewStringUTF(je, "Method returned.");
}
And trying to cause 10MB leak via this method:
Wrapper.causeLeak(10 * 1024 * 1024)
EDIT:
I'm doing this because I want to test my app in a low memory situation.

I couldn't get this working but I found someone on GitHub who built a better approach. If someone ever need to test memory leaks use the repository here: https://github.com/T-Spoon/Android-Developer-Toolbelt

JNI ERROR (app bug): accessed stale local reference 0xbc00021 (index 8 in a table of size 8)

I made hello world application from book Android apps for Absolute Beginners and Temperature Convertor app from here
Both is running fine on Emulator but when I try to run it on Samsung Note 2 following error is coming on LogCat
02-08 07:22:18.665: E/dalvikvm(30944): JNI ERROR (app bug): accessed stale local reference 0xbc00021 (index 8 in a table of size 8)
02-08 07:22:18.665: E/dalvikvm(30944): VM aborting
02-08 07:22:18.665: A/libc(30944): Fatal signal 11 (SIGSEGV) at 0xdeadd00d (code=1), thread 30944 (oid.temperature)
Both applications do open shows layout with title but do not shows any other views in layout
Samples runs fine
device: note 2 Samsung-gt_n7100
IDE:Eclipse version 3.8
OS: 64bit Windows 7

Since android 4.0 garbage collector was changed. Now it moves object around during garbage collection, which can cause a lot of problems.
Imagine that you have a static variable pointing to an object, and then this object gets moved by gc. Since android uses direct pointers for java objects, this would mean that your static variable is now pointing to a random address in the memory, unoccupied by any object or occupied by an object of different sort. This will almost guarantee that you'll get EXC_BAD_ACCESS next time you use this variable.
So android gives you JNI ERROR (app bug) error to prevent you from getting undebugable EXC_BAD_ACCESS. Now there are two ways to avoid this error.
You can set targetSdkVersion in your manifest to version 11 or less. This will enable JNI bug compatibility mode and prevent any problems altogether. This is the reason why your old examples are working.
You can avoid using static variables pointing to java objects or make jobject references global before storing them by calling env->NewGlobalRef(ref).
Perhaps on of the biggest examples here is keeping jclass objects. Normally, you'll initialize static jclass variable during JNI_OnLoad, since class objects remain in the memory as long as the application is running.
This code will lead to a crash:
static jclass myClass;
JNIEXPORT jint JNICALL JNI_OnLoad (JavaVM * vm, void * reserved) {
myClass = env->FindClass("com/example/company/MyClass");
return JNI_VERSION_1_6;
}
While this code will run fine:
static jclass myClass;
JNIEXPORT jint JNICALL JNI_OnLoad (JavaVM * vm, void * reserved) {
jclass tmp = env->FindClass("com/example/company/MyClass");
myClass = (jclass)env->NewGlobalRef(tmp);
return JNI_VERSION_1_6;
}
For more examples see link provided by Marek Sebera: http://android-developers.blogspot.cz/2011/11/jni-local-reference-changes-in-ics.html

android NDK mutex locking

I've been porting a cross platform C++ engine to Android, and noticed that it will inexplicably (and inconsistently) block when calling pthread_mutex_lock. This engine has already been working for many years on several platforms, and the problematic code hasn't changed in years, so I doubt it's a deadlock or otherwise buggy code. It must be my port to Android..
So far there are several places in the code that block on pthread_mutex_lock. It isn't entirely reproducible either. When it hangs, there's no suspicious output in LogCat.
I modified the mutex code like this (edited for brevity... real code checks all return values):
void MutexCreate( Mutex* m )
{
#ifdef WINDOWS
InitializeCriticalSection( m );
#else ANDROID
pthread_mutex_init( m, NULL );
#endif
}
void MutexDestroy( Mutex* m )
{
#ifdef WINDOWS
DeleteCriticalSection( m );
#else ANDROID
pthread_mutex_destroy( m, NULL );
#endif
}
void MutexLock( Mutex* m )
{
#ifdef WINDOWS
EnterCriticalSection( m );
#else ANDROID
pthread_mutex_lock( m );
#endif
}
void MutexUnlock( Mutex* m )
{
#ifdef WINDOWS
LeaveCriticalSection( m );
#else ANDROID
pthread_mutex_unlock( m );
#endif
}
I tried modifying MutexCreate to make error-checking and recursive mutexes, but it didn't matter. I wasn't even getting errors or log output either, so either that means my mutex code is just fine, or the errors/logs weren't being shown. How exactly does the OS notify you of bad mutex usage?
The engine makes heavy use of static variables, including mutexes. I can't see how, but is that a problem? I doubt it because I modified lots of mutexes to be allocated on the heap instead, and the same behavior occurred. But that may be because I missed some static mutexes. I'm probably grasping at straws here.
I read several references including:
http://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_mutex_init.html
http://www.embedded-linux.co.uk/tutorial/mutex_mutandis
http://linux.die.net/man/3/pthread_mutex_init
Android NDK Mutex
Android NDK problem pthread_mutex_unlock issue

The "errorcheck" mutexes will check a couple of things (like attempts to use a non-recursive mutex recursively) but nothing spectacular.
You said "real code checks all return values", so presumably your code explodes if any pthread call returns a nonzero value. (Not sure why your pthread_mutex_destroy takes two args; assuming copy & paste error.)
The pthread code is widely used within Android and has no known hangups, so the issue is not likely in the pthread implementation itself.
The current implementation of mutexes fits in 32 bits, so if you print *(pthread_mutex_t* mut) as an integer you should be able to figure out what state it's in (technically, what state it was in at some point in the past). The definition in bionic/libc/bionic/pthread.c is:
/* a mutex is implemented as a 32-bit integer holding the following fields
*
* bits: name description
* 31-16 tid owner thread's kernel id (recursive and errorcheck only)
* 15-14 type mutex type
* 13 shared process-shared flag
* 12-2 counter counter of recursive mutexes
* 1-0 state lock state (0, 1 or 2)
*/
"Fast" mutexes have a type of 0, and don't set the tid field. In fact, a generic mutex will have a value of 0 (not held), 1 (held), or 2 (held, with contention). If you ever see a fast mutex whose value is not one of those, chances are something came along and stomped on it.
It also means that, if you configure your program to use recursive mutexes, you can see which thread holds the mutex by pulling the bits out (either by printing the mutex value when trylock indicates you're about to stall, or dumping state with gdb on a hung process). That, plus the output of ps -t, will let you know if the thread that locked the mutex still exists.

Running generated ARM machine code on Android gives UnsupportedOperationException with Java Bitmap objects

We ( http://www.mosync.com ) have compiled our ARM recompiler with the Android NDK which takes our internal byte code and generates ARM machine code. When executing recompiled code we see an enormous increase in performance, with one small exception, we can't use any Java Bitmap operations.
The native system uses a function which takes care of all the calls to the Java side which the recompiled code is calling. On the Java (Dalvik) side we then have bindings to Android features. There are no problems while recompiling the code or when executing the machine code. The exact same source code works on Symbian and Windows Mobile 6.x so the recompiler seems to generate correct ARM machine code.
Like I said, the problem we have is that we can't use Java Bitmap objects. We have verified that the parameters which are sent from the Java code is correct, and we have tried following the execution down in Android's own JNI systems. The problem is that we get an UnsupportedOperationException with "size must fit in 32 bits.". The problem seems consistent on Android 1.5 to 2.3. We haven't tried the recompiler on any Android 3 devices.
Is this a bug which other people have encountered, I guess other developers have done similar things.

I found the message in dalvik_system_VMRuntime.c:
/*
* public native boolean trackExternalAllocation(long size)
*
* Asks the VM if <size> bytes can be allocated in an external heap.
* This information may be used to limit the amount of memory available
* to Dalvik threads. Returns false if the VM would rather that the caller
* did not allocate that much memory. If the call returns false, the VM
* will not update its internal counts.
*/
static void Dalvik_dalvik_system_VMRuntime_trackExternalAllocation(
const u4* args, JValue* pResult)
{
s8 longSize = GET_ARG_LONG(args, 1);
/* Fit in 32 bits. */
if (longSize < 0) {
dvmThrowException("Ljava/lang/IllegalArgumentException;",
"size must be positive");
RETURN_VOID();
} else if (longSize > INT_MAX) {
dvmThrowException("Ljava/lang/UnsupportedOperationException;",
"size must fit in 32 bits");
RETURN_VOID();
}
RETURN_BOOLEAN(dvmTrackExternalAllocation((size_t)longSize));
}
This method is called, for example, from GraphicsJNI::setJavaPixelRef:
size_t size = size64.get32();
jlong jsize = size; // the VM wants longs for the size
if (reportSizeToVM) {
// SkDebugf("-------------- inform VM we've allocated %d bytes\n", size);
bool r = env->CallBooleanMethod(gVMRuntime_singleton,
gVMRuntime_trackExternalAllocationMethodID,
jsize);
I would say it seems that the code you're calling is trying to allocate a too big size. If you show the actual Java call which fails and values of all the arguments that you pass to it, it might be easier to find the reason.

I managed to find a work-around. When I wrap all the Bitmap.createBitmap calls inside a Activity.runOnUiThread() It works.