如何合理地估算線程池大小？
這個問題雖然看起來很小，卻並不那麼容易回答。html
你們若是有更好的方法歡迎賜教，先來一個天真的估算方法：java
假設要求一個系統的TPS（Transaction Per Second或者Task Per Second）至少爲20，而後假設每一個Transaction由一個線程完成，繼續假設平均每一個線程處理一個Transaction的時間爲4s。spring
那麼問題轉化爲：如何設計線程池大小，使得能夠在1s內處理完20個Transaction？apache
計算過程很簡單，每一個線程的處理能力爲0.25TPS，那麼要達到20TPS，顯然須要20/0.25=80個線程。服務器
很顯然這個估算方法很天真，由於它沒有考慮到CPU數目。通常服務器的CPU核數爲16或者32，若是有80個線程，那麼確定會帶來太多沒必要要的線程上下文切換開銷。網絡
再來第二種簡單的但不知是否可行的方法（N爲CPU總核數）：多線程
- 若是是CPU密集型應用，則線程池大小設置爲N+1
- 若是是IO密集型應用，則線程池大小設置爲2N+1
若是一臺服務器上只部署這一個應用而且只有這一個線程池，那麼這種估算或許合理，具體還需自行測試驗證。intellij-idea
接下來在這個文檔：服務器性能IO優化 中發現一個估算公式：app
最佳線程數目 = （（線程等待時間+線程CPU時間）/線程CPU時間 ）* CPU數目異步
好比平均每一個線程CPU運行時間爲0.5s，而線程等待時間（非CPU運行時間，好比IO）爲1.5s，CPU核心數爲8，那麼根據上面這個公式估算獲得：((0.5+1.5)/0.5)*8=32。這個公式進一步轉化爲：
最佳線程數目 = （線程等待時間與線程CPU時間之比 + 1）* CPU數目
能夠得出一個結論：線程等待時間所佔比例越高，須要越多線程。線程CPU時間所佔比例越高，須要越少線程。
上一種估算方法也和這個結論相合。
一個系統最快的部分是CPU，因此決定一個系統吞吐量上限的是CPU。加強CPU處理能力，能夠提升系統吞吐量上限。但根據短板效應，真實的系統吞吐量並不能單純根據CPU來計算。那要提升系統吞吐量，就須要從「系統短板」（好比網絡延遲、IO）着手：
- 儘可能提升短板操做的並行化比率，好比多線程下載技術
- 加強短板能力，好比用NIO替代IO
第一條能夠聯繫到Amdahl定律，這條定律定義了串行系統並行化後的加速比計算公式：
加速比=優化前系統耗時 / 優化後系統耗時
加速比越大，代表系統並行化的優化效果越好。Addahl定律還給出了系統並行度、CPU數目和加速比的關係，加速比爲Speedup，系統串行化比率（指串行執行代碼所佔比率）爲F，CPU數目爲N：
Speedup <= 1 / (F + (1-F)/N)
當N足夠大時，串行化比率F越小，加速比Speedup越大。
寫到這裏，我忽然冒出一個問題。
是否使用線程池就必定比使用單線程高效呢？
答案是否認的，好比Redis就是單線程的，但它卻很是高效，基本操做都能達到十萬量級/s。從線程這個角度來看，部分緣由在於：
- 多線程帶來線程上下文切換開銷，單線程就沒有這種開銷
- 鎖
固然「Redis很快」更本質的緣由在於：Redis基本都是內存操做，這種狀況下單線程能夠很高效地利用CPU。而多線程適用場景通常是：存在至關比例的IO和網絡操做。
因此即便有上面的簡單估算方法，也許看似合理，但實際上也未必合理，都須要結合系統真實狀況（好比是IO密集型或者是CPU密集型或者是純內存操做）和硬件環境（CPU、內存、硬盤讀寫速度、網絡情況等）來不斷嘗試達到一個符合實際的合理估算值。
最後來一個「Dark Magic」估算方法（由於我暫時尚未搞懂它的原理），使用下面的類：
package threadpool;
import java.math.BigDecimal;
import java.math.RoundingMode;
import java.util.Timer;
import java.util.TimerTask;
import java.util.concurrent.BlockingQueue;
/**
* A class that calculates the optimal thread pool boundaries. It takes the
* desired target utilization and the desired work queue memory consumption as
* input and retuns thread count and work queue capacity.
*
* @author Niklas Schlimm
*/
public abstract class PoolSizeCalculator {
/**
* The sample queue size to calculate the size of a single {@link Runnable}
* element.
*/
private final int SAMPLE_QUEUE_SIZE = 1000;
/**
* Accuracy of test run. It must finish within 20ms of the testTime
* otherwise we retry the test. This could be configurable.
*/
private final int EPSYLON = 20;
/**
* Control variable for the CPU time investigation.
*/
private volatile boolean expired;
/**
* Time (millis) of the test run in the CPU time calculation.
*/
private final long testtime = 3000;
/**
* Calculates the boundaries of a thread pool for a given {@link Runnable}.
*
* @param targetUtilization the desired utilization of the CPUs (0 <= targetUtilization <= * 1) * @param targetQueueSizeBytes * the desired maximum work queue size of the thread pool (bytes)
*/
protected void calculateBoundaries(BigDecimal targetUtilization, BigDecimal targetQueueSizeBytes) {
calculateOptimalCapacity(targetQueueSizeBytes);
Runnable task = creatTask();
start(task);
start(task); // warm up phase
long cputime = getCurrentThreadCPUTime();
start(task); // test intervall
cputime = getCurrentThreadCPUTime() - cputime;
long waittime = (testtime * 1000000) - cputime;
calculateOptimalThreadCount(cputime, waittime, targetUtilization);
}
private void calculateOptimalCapacity(BigDecimal targetQueueSizeBytes) {
long mem = calculateMemoryUsage();
BigDecimal queueCapacity = targetQueueSizeBytes.divide(new BigDecimal(mem),
RoundingMode.HALF_UP);
System.out.println("Target queue memory usage (bytes): "
+ targetQueueSizeBytes);
System.out.println("createTask() produced " + creatTask().getClass().getName() + " which took " + mem + " bytes in a queue");
System.out.println("Formula: " + targetQueueSizeBytes + " / " + mem);
System.out.println("* Recommended queue capacity (bytes): " + queueCapacity);
}
/**
* Brian Goetz' optimal thread count formula, see 'Java Concurrency in
* * Practice' (chapter 8.2) *
* * @param cpu
* * cpu time consumed by considered task
* * @param wait
* * wait time of considered task
* * @param targetUtilization
* * target utilization of the system
*/
private void calculateOptimalThreadCount(long cpu, long wait,
BigDecimal targetUtilization) {
BigDecimal waitTime = new BigDecimal(wait);
BigDecimal computeTime = new BigDecimal(cpu);
BigDecimal numberOfCPU = new BigDecimal(Runtime.getRuntime()
.availableProcessors());
BigDecimal optimalthreadcount = numberOfCPU.multiply(targetUtilization)
.multiply(new BigDecimal(1).add(waitTime.divide(computeTime,
RoundingMode.HALF_UP)));
System.out.println("Number of CPU: " + numberOfCPU);
System.out.println("Target utilization: " + targetUtilization);
System.out.println("Elapsed time (nanos): " + (testtime * 1000000));
System.out.println("Compute time (nanos): " + cpu);
System.out.println("Wait time (nanos): " + wait);
System.out.println("Formula: " + numberOfCPU + " * "
+ targetUtilization + " * (1 + " + waitTime + " / "
+ computeTime + ")");
System.out.println("* Optimal thread count: " + optimalthreadcount);
}
/**
* * Runs the {@link Runnable} over a period defined in {@link #testtime}.
* * Based on Heinz Kabbutz' ideas
* * (http://www.javaspecialists.eu/archive/Issue124.html).
* *
* * @param task
* * the runnable under investigation
*/
public void start(Runnable task) {
long start = 0;
int runs = 0;
do {
if (++runs > 5) {
throw new IllegalStateException("Test not accurate");
}
expired = false;
start = System.currentTimeMillis();
Timer timer = new Timer();
timer.schedule(new TimerTask() {
public void run() {
expired = true;
}
}, testtime);
while (!expired) {
task.run();
}
start = System.currentTimeMillis() - start;
timer.cancel();
} while (Math.abs(start - testtime) > EPSYLON);
collectGarbage(3);
}
private void collectGarbage(int times) {
for (int i = 0; i < times; i++) {
System.gc();
try {
Thread.sleep(10);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
break;
}
}
}
/**
* Calculates the memory usage of a single element in a work queue. Based on
* Heinz Kabbutz' ideas
* (http://www.javaspecialists.eu/archive/Issue029.html).
*
* @return memory usage of a single {@link Runnable} element in the thread
* pools work queue
*/
public long calculateMemoryUsage() {
BlockingQueue queue = createWorkQueue();
for (int i = 0; i < SAMPLE_QUEUE_SIZE; i++) {
queue.add(creatTask());
}
long mem0 = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
long mem1 = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
queue = null;
collectGarbage(15);
mem0 = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
queue = createWorkQueue();
for (int i = 0; i < SAMPLE_QUEUE_SIZE; i++) {
queue.add(creatTask());
}
collectGarbage(15);
mem1 = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
return (mem1 - mem0) / SAMPLE_QUEUE_SIZE;
}
/**
* Create your runnable task here.
*
* @return an instance of your runnable task under investigation
*/
protected abstract Runnable creatTask();
/**
* Return an instance of the queue used in the thread pool.
*
* @return queue instance
*/
protected abstract BlockingQueue createWorkQueue();
/**
* Calculate current cpu time. Various frameworks may be used here,
* depending on the operating system in use. (e.g.
* http://www.hyperic.com/products/sigar). The more accurate the CPU time
* measurement, the more accurate the results for thread count boundaries.
*
* @return current cpu time of current thread
*/
protected abstract long getCurrentThreadCPUTime();
}
而後本身繼承這個抽象類並實現它的三個抽象方法，好比下面是我寫的一個示例（任務是請求網絡數據），其中我指按期望CPU利用率爲1.0（即100%），任務隊列總大小不超過100,000字節：
package threadpool;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.lang.management.ManagementFactory;
import java.math.BigDecimal;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
public class SimplePoolSizeCaculatorImpl extends PoolSizeCalculator {
@Override
protected Runnable creatTask() {
return new AsyncIOTask();
}
@Override
protected BlockingQueue createWorkQueue() {
return new LinkedBlockingQueue(1000);
}
@Override
protected long getCurrentThreadCPUTime() {
return ManagementFactory.getThreadMXBean().getCurrentThreadCpuTime();
}
public static void main(String[] args) {
PoolSizeCalculator poolSizeCalculator = new SimplePoolSizeCaculatorImpl();
poolSizeCalculator.calculateBoundaries(new BigDecimal(1.0), new BigDecimal(100000));
}
}
/**
* 自定義的異步IO任務
* @author Will
*
*/
class AsyncIOTask implements Runnable {
public void run() {
HttpURLConnection connection = null;
BufferedReader reader = null;
try {
String getURL = "http://baidu.com";
URL getUrl = new URL(getURL);
connection = (HttpURLConnection) getUrl.openConnection();
connection.connect();
reader = new BufferedReader(new InputStreamReader(
connection.getInputStream()));
String line;
while ((line = reader.readLine()) != null) {
// empty loop
}
}
catch (IOException e) {
} finally {
if(reader != null) {
try {
reader.close();
}
catch(Exception e) {
}
}
connection.disconnect();
}
}
}
獲得以下輸出：
Target queue memory usage (bytes): 100000 createTask() produced threadpool.AsyncIOTask which took 40 bytes in a queue Formula: 100000 / 40 * Recommended queue capacity (bytes): 2500 Number of CPU: 8 Target utilization: 1 Elapsed time (nanos): 3000000000 Compute time (nanos): 280801800 Wait time (nanos): 2719198200 Formula: 8 * 1 * (1 + 2719198200 / 280801800) * Optimal thread count: 88
推薦的任務隊列大小爲2500，線程數爲88。依次爲依據，咱們就能夠構造這樣一個線程池：
ThreadPoolExecutor pool = new ThreadPoolExecutor(88, 88, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(2500));
能夠將這個文件打包成可執行的jar文件，這樣就能夠拷貝到測試/正式環境上執行。
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>threadpool</groupId>
<artifactId>dark-magic</artifactId>
<version>1.0-SNAPSHOT</version>
<packaging>jar</packaging>
<name>dark_magic</name>
<url>http://maven.apache.org</url>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
</properties>
<dependencies>
</dependencies>
<build>
<finalName>dark-magic</finalName>
<plugins>
<plugin>
<artifactId>maven-assembly-plugin</artifactId>
<configuration>
<appendAssemblyId>false</appendAssemblyId>
<descriptorRefs>
<descriptorRef>jar-with-dependencies</descriptorRef>
</descriptorRefs>
<archive>
<manifest>
<!-- 此處指定main方法入口的class -->
<mainClass>threadpool.SimplePoolSizeCaculatorImpl</mainClass>
</manifest>
</archive>
</configuration>
<executions>
<execution>
<id>make-assembly</id>
<phase>package</phase>
<goals>
<goal>assembly</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
</project>
來源：
www.cnblogs.com/cjsblog/p/9068886.html
參考：
http://ifeve.com/how-to-calculate-threadpool-size/
http://www.importnew.com/17384.html
http://www.javashuo.com/article/p-qmnpuqjz-oa.html
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