Re: Java driver connection pool degradation

[Rob Moore]

Kevin -

What is it that is being timed?  The length of the operation or getting the Mongo instance? Does each thread perform a single operation and then die or does it perform multiple operations in a loop?

Rob.

[Kevin B]

Hi Rob-

The operation of getting the Mongo instance is being timed and yes each thread performs a single operation.

I have attached code for you to look at.

Kevin

[Rob Moore]

Do you get the same result for these two versions?

            Mongo mongo = MongoFactory.getInstance(); 
            //Mongo mongo = new Mongo("localhost", op);

I'm assuming the MongoFactory.getInstance() method is synchronized?  If not I think it should be.  If there is a synchronized block I'm not sure what is happening.  If there isn't then it degrades into the "new Mongo" case..

For the "new Mongo" case I think you are probably just running into a self inflicted denial-of-service against the mongo servers.  You are attempting to open 500 connections to the server all at once. 

Each Mongo instance is a logical connection to the server that will multiplex over many physical connections so the MongoFactory.getInstance() is the "right" thing to do.  Based on you previous code I'd expect the getInstance() method to look like:

private static Mongo mongo = null;
public static synchronized Mongo getInstance() {
    if (mongo == null) {

        try {
            MongoOptions op = new MongoOptions();
            op.connectionsPerHost = 100;
            op.threadsAllowedToBlockForConnectionMultiplier = 1500;
            mongo = new Mongo("127.0.0.1", op);              
       } catch (UnknownHostException ex) {
            Logger.getLogger(MongoFactory.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    return mongo;

}

Rob.

[Kevin B]

Hi Rob-
Thanks for the feedback. I added in the "synchronized" to the static method and it helped some, but it still shows degradation. Below are the new execution times. This simulates about 500 transactions a second. I figured Mongo could handle this kind of load, but maybe I'm misunderstanding something.

Thanks
Kevin

connection Execution time was 78 ms. thread 0
connection Execution time was 62 ms. thread 30
connection Execution time was 62 ms. thread 26
connection Execution time was 62 ms. thread 29
connection Execution time was 62 ms. thread 25
connection Execution time was 62 ms. thread 24
connection Execution time was 62 ms. thread 32
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connection Execution time was 62 ms. thread 35
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connection Execution time was 62 ms. thread 21
connection Execution time was 62 ms. thread 36
connection Execution time was 62 ms. thread 20
connection Execution time was 62 ms. thread 28
connection Execution time was 78 ms. thread 19
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connection Execution time was 78 ms. thread 18
connection Execution time was 78 ms. thread 42
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connection Execution time was 93 ms. thread 15
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connection Execution time was 109 ms. thread 6
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connection Execution time was 109 ms. thread 1
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[Rob Moore]

On Sunday, November 4, 2012 5:14:59 PM UTC-5, Kevin B wrote:

Hi Rob-

Thanks for the feedback. I added in the "synchronized" to the static method and it helped some, but it still shows degradation. Below are the new execution times. This simulates about 500 transactions a second. I figured Mongo could handle this kind of load, but maybe I'm misunderstanding something.

Based on where you have the start and end times (around MongoFactory.getInstance()) you are not really capturing to create and/or send a request but just the time for each thread (except the first) to grab the lock, return the Mongo reference and release the lock when the lock is highly contended (i.e., slow).

There are tricks to make grabbing the Mongo reference faster[1] but none have much to do with the performance of MongoDB or the driver.

Rob.


[1] The easiest is to pre-allocate the Mongo reference before the test run and then remove the synchronized key word from the method. The easiest way to make the Mongo reference is to call the MongoFactory.getInstance() as the first thing in the main().

Another is the often maligned double check lock.  I'm not sure the double check will help because I think all of the threads are started and waiting before the first thread has finished creating the initial Mongo instance.

[Kevin B]

Hi Rob-

That did the trick!

Added below code to my test main, and now execution time is 0. Thank you for taking the time on this!

Mongo mongo = MongoFactory.getInstance();
       
        for (int i = 0; i < 1000; i++) {


            Thread thread1 = new Thread(new RunnableThread(), "thread " + i);


            thread1.start();