Re: Thread overhead

" opalinski from opalpaweb" <>
29 Sep 2006 05:50:37 -0700
Let me add a little to what Matt, hiwa, and EJB have stated:

EJB's suggestion to look into java.util.Timer is very good. Since you
find original design incredibly easy to do you may be suprised that
there is an easier way still.

Specific item to keep in mind: When your program grows to monitor a
hundred or two hundred things you may run into a file descriptor
shortage. This situation is from perosnal experience with having two
hundred threads each of which had an open socket. File descriptor
limits are system attributes, not Java's.

Thread weight varies greatly. In some environments threads are very
cheap, and incredibly fast. In some environment threads are more than
ten times more expensive (in terms of time to create and memory
needed). In some environments you can choose a model when starting a
VM. The available thread models have changed with jdk versions. It
may be prudent to look into thread models on the system and jdks you
use and will be using in the future. For example for Solaris you can
learn about your threading options at . Google helps
alot in researching Java's threading models.

Last comment, I second hiwa's suggestion to look into wait/notify .
I'm not sure where you are in your Java experience. wait/notify are
not the simplest things to get comfortable with. But look, every
Object in java has wait/notify, it's as ubiqitous as equals, hashCode,
and toString .

All the best,

Simon Brooke wrote:

I'm developing an abstract monitoring framework which consists of a
(potentially) large number of simple agents each of which watches a single
thing, where these things change slowly and infrequently. My Watcher
objects sleep for periods typically from a few hours up to many days, wake
up, check the thing they're watching in a few seconds, and then if
everything's OK go to sleep again. In implementation terms it's incredibly
easy to do this by giving each Watcher a thread of its own:

    /** a token to identify the sleep time in the configuration */
    public static final String INTERVALTOKEN = "period";

    /** how many milliseconds I sleep between periods of activity */
    protected long winks = 5 * 60 * 1000;

    /** my thread */
    protected Thread thread = null;

    /** the exceptions which occurred as I was watching */
    protected Vector whinges = new Vector( );

     * initialise me with this configuration
    public void init( Map config ) throws AlertingException
        Object val = config.get( INTERVALTOKEN );

        if ( val == null )
            val = config.get( "p" );

        if ( val != null )
            winks = (long) ( Integer.parseInt( val.toString( ) ) * 1000 );

     * perform a check to see whether the event I am watching for has
     * occurred.
     * @return a vector of maps each one of which represents an alert which
     * should be sent. If no alerts should be sent, return an empty
     * vector.
    protected abstract Vector check( ) throws Exception;

     * periodically wake up check the things I'm watching. If any have
     * changed send appropriate alerts
    public void run( )
        while ( thread != null )
            if ( whinges.size( ) < TOOMANYERRORS)
                    Vector alerts = check( );

                    if ( alerts != null )
                        Enumeration e = alerts.elements( );

                        while ( e.hasMoreElements( ) )
                            Map context = (Map) e.nextElement( );

                                sendAlert( context );
                                mark( context );
                            catch ( Exception oops )
                                whinge( oops );
                                whinges.add( oops );
                catch ( Exception whinge )
                    whinge( whinge );
                    whinges.add( whinge );
            { // stop the thread!
                stop( );

                Thread.sleep( winks );
            catch ( InterruptedException wakeywakey )

But what's bothering me is what this costs. It particularly bothers me
because, while at this stage in the development of my system I'm dealing
with a few tens of these objects, in the long term I could be dealing with
several hundreds.

Are threads essentially very lightweight things which allow this framework
to be efficient, or are they quite heavyweight and I'd be better working
on an architecture where a single thread wakes up periodically and polls
watchers for whether they want to run?

-- (Simon Brooke)
Ye hypocrites! are these your pranks? To murder men and give God thanks?
Desist, for shame! Proceed no further: God won't accept your thanks for
            -- Robert Burns, 'Thanksgiving For a National Victory'

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