Alex Rivera | Logout

AsyncTask, must it take such a performance penalty hit...?

Asked 2012-01-21T18:21:53.493
29

I'm developing a small app that reads in specific html-pages, re-formats them and then shows them in a WebView. If I run my code in the GUI thread, the performance hit is close to negligible compared to simply letting the WebView show the original html-page. But if I'm a good boy and do like I'm told, I'm supposed to use an AsyncTask to run the code in the background so as not to freeze up the GUI during those 3-5 seconds my code does its job. Problem is... if I do so, the code takes more than 10 times as long to finish. A page takes 60+ seconds to show, which is unacceptable.

Tracking down the problem, TraceView shows me that my AsyncTask is (at default priority) run in roughly 10 ms chunks, around 4 times per second. I need to set my thread priority to MAX_PRIORITY to get close to acceptable loading times, but even then it takes 3-4 times longer than when I run in the GUI thread.

Am I doing something wrong, or is this just the way it works? And must it work this way...?

Here's compilable code as requested:

package my.ownpackage.athome;

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.StrictMode;
import android.webkit.WebView;
import android.webkit.WebViewClient;

public class AndroidTestActivity extends Activity
{   
    WebView webview;
    //...

    private class HelloWebViewClient extends WebViewClient 
    {
        @Override
        public boolean shouldOverrideUrlLoading(WebView view, String url) 
        {
            AndroidTestActivity.this.fetch(view, url);
            return true;
        }
    }

    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        // To allow to connect to the web and pull down html-files, reset strict mode
        // see http://stackoverflow.com/questions/8706464/defaulthttpclient-to-androidhttpclient
        if (android.os.Build.VERSION.SDK_
android android-asynctask
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1 Answer

37

You're not the only one observing this behaviour. The slowdown by factor 10 is probably a result of Android using a Linux cgroup (scheduling class) for threads of priority BACKGROUND or below. All these threads have to live with 10% CPU time altogether.

The good news is you don't have to live with the Thread priority settings from java.lang.Thread. You can assign your Thread a pthread (Linux thread) priority from the definitions in android.os.Process. There, you not only have Process.THREAD_PRIORITY_BACKGROUND, but also constants to adjust the priority a bit.

Currently, Android uses the background thread cgroup for all threads with priority THREAD_PRIORITY_BACKGROUND or worse, and THREAD_PRIORITY_BACKGROUND is 10 while THREAD_PRIORITY_DEFAULT is 0 and THREAD_PRIORITY_FOREGROUND is -2.

If you go for THREAD_PRIORITY_BACKGROUND + THREAD_PRIORITY_MORE_FAVORABLE (aka 9) your thread will be lifted out of the background cgroup with the 10% limitation, while not being important enough to interrupt your User Interface threads too often.

I believe there are background tasks which need a bit of computational power but which are at the same time not important enough to de facto block the UI (by consuming too much CPU in a separate thread) and Android currently has no obvious priority to assign to these, so in my view, this is one of the best priorities you can assign to such a task.

If you can use a HandlerThread it's easy to achieve:

ht = new HandlerThread("thread name", THREAD_PRIORITY_BACKGROUND + THREAD_PRIORITY_MORE_FAVORABLE);
ht.start();
h  = new Handler(ht.getLooper());

If you want to go with AsyncTask, you can still do

protected final YourResult doInBackground(YourInputs... yis) {
    Process.setThreadPriority(THREAD_PRIORITY_BACKGROUND + THREAD_PRIORITY_MORE_FAVORABLE);
    ...
}

but be aware that the underlying implementation may reuse the same Thread objec

answered 2013-01-08T15:01:15.260

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