Reasonably configure thread pool

Reasonable configuration of thread pool

To properly configure the thread pool , We must first analyze the task characteristics , It can be analyzed from the following aspects .

• Nature of task ：CPU Intensive task 、IO Intensive tasks and hybrid tasks .

• Task priority ： high 、 And low .

• Task execution time ： Long 、 And short .

• Task dependency ： Whether to rely on other system resources , Such as database connection .

Tasks of different nature can be handled separately by thread pools of different sizes .

CPU Intensive tasks should be configured with threads as small as possible , Such as configuration N*cpu+1 A pool of threads .

IO Intensive task threads do not always perform tasks , Configure as many threads as possible , Such as 2*Ncpu.

Hybrid tasks , If you can split , Split it into one CPU Intensive tasks and a IO Intensive task , As long as the time difference between the two tasks is not too large , Then the throughput of execution after decomposition will be higher than that of serial execution . If the time difference between the two tasks is too large , No need to decompose .

Can pass Runtime.getRuntime().availableProcessors() Method to get the CPU Number .

Priority queues can be used for tasks with different priorities PriorityBlockingQueue To deal with it . It allows high priority tasks to be executed first .

Be careful   If there are always high priority tasks submitted to the queue , Then tasks with low priority may never be performed .

Tasks with different execution times can be handed over to different scale thread pools for processing , Or you can use priority queues , Let tasks with short execution time be executed first .

Tasks that depend on the database connection pool , Because thread commit SQL After that, you need to wait for the database to return the result , The longer you wait , be CPU The longer the free time , Then the more threads should be set , Only in this way can we make better use of CPU. Bounded queues are recommended . Bounded queues can increase the stability and early warning ability of the system , You can make it bigger as needed , Like thousands . There is a , The queue and thread pool of background task thread pool in our system are full , Keep throwing exceptions that discard tasks , Through investigation, we found that there was a problem with the database , Lead to execution SQL It's getting very slow , Because all tasks in the background task thread pool need to query and insert data into the database , So all the worker threads in the thread pool are blocked , Task backlog in the process pool . If we set up an unbounded queue at that time , Then there will be more and more queues in the thread pool , It's possible to fill up the memory , Make the whole system unavailable , It's not just background tasks that have problems . Of course , All the tasks of our system are deployed with a separate server , We use thread pools of different sizes to accomplish different types of tasks , But this kind of problem will also affect other tasks .

Thread pool monitoring

If the thread pool is widely used in the system , It is necessary to monitor the thread pool , Convenient in case of problems , It can quickly locate the problem according to the usage of the thread pool . It can be monitored by the parameters provided by the thread pool , The following properties can be used when monitoring the thread pool .

• taskCount： The number of tasks that the thread pool needs to perform .

• completedTaskCount： The number of tasks completed by the thread pool during the run , Less than or equal to taskCount.

• largestPoolSize： Maximum number of threads ever created in the thread pool . Through this data, we can know whether the thread pool has been full . If the value is equal to the maximum size of the thread pool , Indicates that the thread pool has been full .

• getPoolSize： Number of threads in the thread pool . If the thread pool is not destroyed , Threads in the thread pool will not be destroyed automatically , So this size just keeps growing .

• getActiveCount： Get the number of active threads .

Monitoring by expanding the thread pool . You can customize the thread pool by inheriting it , Rewrite thread pool beforeExecute、afterExecute and terminated Method , Or before the task is executed 、 After execution and before the thread pool is closed, execute some code to monitor . for example , Average execution time of monitoring tasks 、 Maximum execution time and minimum execution time, etc .