What causes the periodic performance spikes that are seen when doing computationally expensive array processing on the Nexus 4?

StackOverflow https://stackoverflow.com/questions/14299288

Question

I'm new to threads (don't kill me for my implementation below :) and I need to do multiple blurring passes of pixels on a separate thread (see below). It's not the most efficient implementation of box blur (it's from Gaussian Filter without using ConvolveOp) but the performance spikes don't occur on the Nexus 7 tablet but they do occur on the Nexus 4 phone.

I've posted my testing sample (running on Android 4.2 - see below).

I don't think it is caused by the GC thrashing the memory (it doesn't coincide with the spikes).

I think it might be something to do with cache locality or hardware memory thrashing - but I'm not sure.

What would cause the spikes? Sometimes they are sudden onset - e.g. spike of 50%. Sometimes they are slow onset - e.g. spikes increasing/decreasing monotonically, with spikes as follows -> 5%, 10%, 20%, 10%, 5%.

How could I stop them from occurring when doing heavy array processing?

This doesn't occur on the Nexus 7 tablet which I have also tested (see results below)

Side question: What is the best way to sleep and restart my thread correctly (new to threads)?

MainActivity.java

package com.example.test;

import android.os.Bundle;
import android.app.Activity;

public class MainActivity extends Activity {

    private MainThread thread;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        thread = new MainThread();

        thread.setRunning(true);
        thread.start();

        setContentView(R.layout.activity_main);
    }

    @Override
    protected void onResume() {

        super.onResume();
        thread.setRunning(true);

    }

    @Override
    protected void onPause() {

        super.onPause();
        thread.setRunning(false);

    }

}

MainThread.java

package com.example.test;

import android.util.Log;

public class MainThread extends Thread {

    int[] pixels;
    int kernel_rows = 2;
    int kernel_cols = 2;
    int width = 512;
    int height = 512;

    @Override
    public void run() {

        while (running) {

            long start = System.currentTimeMillis();

            for (int row = kernel_rows / 2; row < height - kernel_rows / 2; row++) {
                for (int col = kernel_cols / 2; col < width - kernel_cols / 2; col++) {

                    float pixel = 0;

                    // iterate over each pixel in the kernel
                    for (int row_offset = 0; row_offset < kernel_rows; row_offset++) {
                        for (int col_offset = 0; col_offset < kernel_cols; col_offset++) {

                            // subtract by half the kernel size to center the
                            // kernel
                            // on the pixel in question
                            final int row_index = row + row_offset
                                    - kernel_rows / 2;
                            final int col_index = col + col_offset
                                    - kernel_cols / 2;

                            pixel += pixels[row_index * width + col_index] * 1.0f / 4.0f;

                        }
                    }

                    pixels[row * width + col] = (int) pixel;

                }

            }

            long stop = System.currentTimeMillis();

            long delta = stop - start;

            Log.d("DELTA", Long.toString(delta));

        }

    }

    private boolean running;

    public void setRunning(boolean running) {

        this.pixels = new int[512 * 512];
        this.running = running;

    }

}

Logs

Nexus 4 phone (ms):

01-13 10:56:05.663: D/DELTA(13507): 76
01-13 10:56:05.773: D/DELTA(13507): 107
01-13 10:56:05.843: D/DELTA(13507): 77
01-13 10:56:05.923: D/DELTA(13507): 75
01-13 10:56:06.053: D/DELTA(13507): 127
01-13 10:56:06.133: D/DELTA(13507): 78
01-13 10:56:06.213: D/DELTA(13507): 81
01-13 10:56:06.293: D/DELTA(13507): 80
01-13 10:56:06.353: D/DELTA(13507): 77
01-13 10:56:06.433: D/DELTA(13507): 79
01-13 10:56:06.513: D/DELTA(13507): 79
01-13 10:56:06.624: D/DELTA(13507): 106
01-13 10:56:06.694: D/DELTA(13507): 76

Nexus 7 tablet (ms):

01-13 11:01:03.283: D/DELTA(3909): 84
01-13 11:01:03.373: D/DELTA(3909): 85
01-13 11:01:03.453: D/DELTA(3909): 85
01-13 11:01:03.543: D/DELTA(3909): 84
01-13 11:01:03.623: D/DELTA(3909): 85
01-13 11:01:03.703: D/DELTA(3909): 84
01-13 11:01:03.793: D/DELTA(3909): 85
01-13 11:01:03.873: D/DELTA(3909): 84
01-13 11:01:03.963: D/DELTA(3909): 85
01-13 11:01:04.043: D/DELTA(3909): 84
Was it helpful?

Solution

I think I may have somewhat mitigated this effect on the Nexus 4. There is still some variability in calculation consistency but it is bearable - I think - can't see too many huge spikes - outside of thread startup/shutdown. I've done this using the Android NDK and c p_threads to spawn a native thread that is mostly left alone by Java (or so I'm told) until the foreground application is changed or closed.

Here is the code:

MainActivity.java

package com.example.test;

import android.os.Bundle;
import android.app.Activity;

public class MainActivity extends Activity {

    static {

        System.loadLibrary("native");

    }

    private native void init();

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        // Initializes and spawns native thread
        init();

        setContentView(R.layout.activity_main);

    }

}

native.c

(which should be put into a jni folder at the root of the Android project)

#include <time.h>
#include <pthread.h>
#include <jni.h>
#include <android/log.h>

#define APPNAME "DELTA"

int* pixels;
int kernel_rows = 2;
int kernel_cols = 2;
int width = 60;
int height = 39;

int running = 1;

// from android samples
/* return current time in milliseconds */
static double now_ms(void) {

    struct timespec res;
    clock_gettime(CLOCK_REALTIME, &res);
    return 1000.0 * res.tv_sec + (double) res.tv_nsec / 1e6;

}

// initialize thread/begin it
jint Java_com_example_testa_MainActivity_init(JNIEnv* env, jobject javaThis) {

    int i1 = 1;

    pthread_t thread;
    void *run();

    pthread_create(&thread, NULL, run, &i1);
    pthread_join(thread, NULL);

    return 0;

}

// thread function
void *run(int *x) {

    // init pixels within thread
    pixels = (int*) malloc(sizeof(int) * width * height);

    // loop until stopped - java won't interfere
    // unless closed/switch application (or so I'm told)
    while (running) {

        double start = now_ms();

        int row, col, row_offset, col_offset;

        for (row = kernel_rows / 2; row < height - kernel_rows / 2; row++) {

            for (col = kernel_cols / 2; col < width - kernel_cols / 2; col++) {

                float pixel = 0;

                // iterate over each pixel in the kernel
                for (row_offset = 0; row_offset < kernel_rows; row_offset++) {
                    for (col_offset = 0; col_offset < kernel_cols;
                            col_offset++) {

                        // subtract by half the kernel size to center the
                        // kernel
                        // on the pixel in question

                        int row_index = row + row_offset - kernel_rows / 2;
                        int col_index = col + col_offset - kernel_cols / 2;

                        pixel += pixels[row_index * width + col_index] * 1.0f
                                / 4.0f;

                    }

                }

                pixels[row * width + col] = (int) pixel;

            }

        }

        double end = now_ms();

        double delta = end - start;

        __android_log_print(ANDROID_LOG_VERBOSE, APPNAME, "%f", delta);

    }

    pthread_exit(0);

}

Android.mk

(which should be put into a jni folder at the root of the Android project)

LOCAL_PATH := $(call my-dir)
MY_PATH := $(LOCAL_PATH)
include $(call all-subdir-makefiles)

include $(CLEAR_VARS)

LOCAL_PATH := $(MY_PATH)

LOCAL_MODULE    := native
LOCAL_LDLIBS := -llog
LOCAL_SRC_FILES := native.c

include $(BUILD_SHARED_LIBRARY)

Summary

Reduced cost of code by ~20-30% and decreased variability by an order of magnitude.

The code is compiled by executing the ndk-build command from the NDK library provided by Android at the root folder (found here: http://developer.android.com/tools/sdk/ndk/index.html).

Results

Nexus 4 (ms):

01-14 13:41:21.132: V/DELTA(23679): 56.554199
01-14 13:41:21.192: V/DELTA(23679): 58.568604
01-14 13:41:21.252: V/DELTA(23679): 59.484131
01-14 13:41:21.302: V/DELTA(23679): 56.768066
01-14 13:41:21.362: V/DELTA(23679): 54.692383
01-14 13:41:21.412: V/DELTA(23679): 51.823730
01-14 13:41:21.472: V/DELTA(23679): 55.668945
01-14 13:41:21.522: V/DELTA(23679): 56.920654
01-14 13:41:21.582: V/DELTA(23679): 56.371094
01-14 13:41:21.642: V/DELTA(23679): 58.507568
01-14 13:41:21.702: V/DELTA(23679): 59.697754
01-14 13:41:21.752: V/DELTA(23679): 53.990723
01-14 13:41:21.812: V/DELTA(23679): 55.669189

Nexus 7 (ms):

01-14 13:41:25.685: V/DELTA(2916): 65.867920
01-14 13:41:25.745: V/DELTA(2916): 65.986816
01-14 13:41:25.815: V/DELTA(2916): 66.685059
01-14 13:41:25.885: V/DELTA(2916): 67.033936
01-14 13:41:25.945: V/DELTA(2916): 65.703857
01-14 13:41:26.015: V/DELTA(2916): 66.653076
01-14 13:41:26.085: V/DELTA(2916): 66.922119
01-14 13:41:26.145: V/DELTA(2916): 67.030029
01-14 13:41:26.215: V/DELTA(2916): 67.014893
01-14 13:41:26.285: V/DELTA(2916): 67.034912
01-14 13:41:26.345: V/DELTA(2916): 67.089844
01-14 13:41:26.415: V/DELTA(2916): 65.860107
01-14 13:41:26.485: V/DELTA(2916): 65.642090
01-14 13:41:26.545: V/DELTA(2916): 65.574951
01-14 13:41:26.615: V/DELTA(2916): 65.991943
Licensed under: CC-BY-SA with attribution
Not affiliated with StackOverflow
scroll top