在这种情况下，多线程如何帮助提高性能？

Question

I have a piece of code like this:

while(){

    x = jdbc_readOperation();

    y = getTokens(x);

    jdbc_insertOperation(y);
}

public List<String> getTokens(String divText){
    List<String> tokenList = new ArrayList<String>();
    Matcher subMatcher = Pattern.compile("\\[[^\\]]*]").matcher(divText);
    while (subMatcher.find()) {
        String token = subMatcher.group();
        tokenList.add(token);
    }
    return tokenList;
}

What I know is using multithreading can save time when one thread is get blocked by I/O or network. In this synchronous operations every step have to wait for its previous step get finished. What I want here is to maximize cpu utilization on getTokens() .

My first thought is put getTokens() in the run method of a class, and create multiple threads. But I think it will not work since it seems not able to get performance benefit by having multiple threads on pure computation operations.

Is adoption of multithreading going to help increase performance in this case? If so, how can I do that?

Answer 1

It will depend on the pace that jdbc_readOperation() produces data to be processed in comparison with the pace that getTokens(x) processes the data. Knowing that will help you figure out if multi-threading is going to help you.

You could try something like this (just for you to get the idea):

int workToBeDoneQueueSize = 1000;
int workDoneQueueSize = 1000;
BlockingQueue<String> workToBeDone = new LinkedBlockingQueue<>(workToBeDoneQueueSize);
BlockingQueue<String> workDone = new LinkedBlockingQueue<>(workDoneQueueSize);

new Thread(() -> {
    try {
        while (true) {
            workToBeDone.put(jdbc_readOperation());
        }
    } catch (InterruptedException e) {
        e.printStackTrace();
        // handle InterruptedException here
    }
}).start();

int numOfWorkerThreads = 5; // just an example
for (int i = 0; i < numOfWorkerThreads; i++) {
    new Thread(() -> {
        try {
            while (true) {
                workDone.put(getTokens(workToBeDone.take()));
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
            // handle InterruptedException here
        }
    }).start();
}

new Thread(() -> {
    // you could improve this by making a batch operation
    try {
        while (true) {
            jdbc_insertOperation(workDone.take());
        }
    } catch (InterruptedException e) {
        e.printStackTrace();
        // handle InterruptedException here
    }
}).start();

Or you could learn how to use the ThreadPoolExecutor. ( https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ThreadPoolExecutor.html )

Answer 2

Okay to speed up getTokens() you can split the inputted String divText by using String.substring() method. You split it into as many substrings as you will run Threads running the getTokens() method. Then every Thread will "run" on a certain substring of divText.

Creating more Threads than the CPU can handle should be avoided since context switches create inefficiency.

https://docs.oracle.com/javase/8/docs/api/java/lang/String.html#substring-int-int-

An alternative could be splitting the inputted String of getTokens with the String.split method http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#split%28java.lang.String%29 eg in case the text is made up of words seperated by spaces or other symbols. Then specific parts of the resulting String array could be passed to different Threads.