不同并發(fā)場(chǎng)景下LongAdder與AtomicLong如何選擇

點(diǎn)擊上方“碼農(nóng)突圍”，馬上關(guān)注
這里是碼農(nóng)充電第一站，回復(fù)“666”，獲取一份專屬大禮包
真愛，請(qǐng)?jiān)O(shè)置“星標(biāo)”或點(diǎn)個(gè)“在看

| 寫在前面

| volatile

public?class?VolatileTest?{
????private?static?final?int?THREAD_COUNT?=?20;

????private?static?volatile?int?race?=?0;

????public?static?void?increase()?{
????????race++;
????}

????public?static?void?main(String[]?args)?{
????????Thread[]?threads?=?new?Thread[THREAD_COUNT];
????????for?(int?i?=?0;?i?????????????threads[i]?=?new?Thread(new?Runnable()?{
????????????????@Override
????????????????public?void?run()?{
????????????????????for?(int?i?=?0;?i?1000;?i++)?{
????????????????????????increase();
????????????????????}
????????????????}
????????????});
????????????threads[i].start();
????????}

????????//等所有累加線程都結(jié)束
????????while?(Thread.activeCount()?>?1)?{
????????????Thread.yield();
????????}

????????System.out.println("race:?"?+?race);
????}
}

public?class?AtomicLongTest?{
????private?static?final?int?THREAD_COUNT?=?20;

????private?static?volatile?AtomicLong?race?=?new?AtomicLong(0);

????public?static?void?increase()?{
????????race.getAndIncrement();
????}

????public?static?void?main(String[]?args)?{
????????Thread[]?threads?=?new?Thread[THREAD_COUNT];
????????for?(int?i?=?0;?i?????????????threads[i]?=?new?Thread(new?Runnable()?{
????????????????@Override
????????????????public?void?run()?{
????????????????????for?(int?i?=?0;?i?1000;?i++)?{
????????????????????????increase();
????????????????????}
????????????????}
????????????});
????????????threads[i].start();
????????}

????????//等所有累加線程都結(jié)束
????????while?(Thread.activeCount()?>?1)?{
????????????Thread.yield();
????????}

????????System.out.println("race:?"?+?race);
????}
}
運(yùn)算后得到了預(yù)期結(jié)果：20000。

| LongAdder

public?class?LongAdderTest?{
????private?static?final?int?THREAD_COUNT?=?20;

????//默認(rèn)初始化為0值
????private?static?volatile?LongAdder?race?=?new?LongAdder();

????public?static?void?increase()?{
????????race.increment();
????}

????public?static?void?main(String[]?args)?{
????????Thread[]?threads?=?new?Thread[THREAD_COUNT];
????????for?(int?i?=?0;?i?????????????threads[i]?=?new?Thread(new?Runnable()?{
????????????????@Override
????????????????public?void?run()?{
????????????????????for?(int?i?=?0;?i?1000;?i++)?{
????????????????????????increase();
????????????????????}
????????????????}
????????????});
????????????threads[i].start();
????????}

????????while?(Thread.activeCount()?>?1)?{
????????????Thread.yield();
????????}

????????System.out.println("race:?"?+?race);
????}
}
結(jié)果也是預(yù)期的20000。

| AtomicLong和LongAdder性能比較

@BenchmarkMode(Mode.Throughput)
@OutputTimeUnit(TimeUnit.MILLISECONDS)
public?class?PerformaceTest?{
????private?static?AtomicLong?atomicLong?=?new?AtomicLong();
????private?static?LongAdder?longAdder?=?new?LongAdder();

????@Benchmark
????@Threads(10)
????public?void?atomicLongAdd()?{
????????atomicLong.getAndIncrement();
????}

????@Benchmark
????@Threads(10)
????public?void?longAdderAdd()?{
????????longAdder.increment();
????}

????public?static?void?main(String[]?args)?throws?RunnerException?{
????????Options?options?=?new?OptionsBuilder().include(PerformaceTest.class.getSimpleName()).build();
????????new?Runner(options).run();
????}
}
說明：

• @BenchmarkMode(Mode.Throughput) => 測(cè)試吞吐量
• @OutputTimeUnit(TimeUnit.MILLISECONDS) => 輸出的時(shí)間單位
• @Threads(10) => 每個(gè)進(jìn)程中的測(cè)試線程數(shù)

Benchmark??????????????????????Mode??Cnt???????Score?????Error???Units
PerformaceTest.atomicLongAdd??thrpt??200??153824.699?±?137.947??ops/ms
PerformaceTest.longAdderAdd???thrpt??200??124087.220?±??81.015??ops/ms
線程數(shù)為5：

PerformaceTest.atomicLongAdd??thrpt??200???56392.136?±?1165.361??ops/ms
PerformaceTest.longAdderAdd???thrpt??200??605501.870?±?4140.190??ops/ms
線程數(shù)為10：

Benchmark??????????????????????Mode??Cnt???????Score??????Error???Units
PerformaceTest.atomicLongAdd??thrpt??200???53286.334?±??957.765??ops/ms
PerformaceTest.longAdderAdd???thrpt??200??713884.602?±?3950.884??ops/ms
從測(cè)試結(jié)果來看，當(dāng)線程數(shù)為5時(shí)，LongAdder的性能已經(jīng)優(yōu)于AtomicLong。

| 產(chǎn)生性能差異的原因

AtomicLong#getAndIncrement方法分析

//AtomicLong#getAndIncrement
public?final?long?getAndIncrement()?{
????return?unsafe.getAndAddLong(this,?valueOffset,?1L);
}

//Unsafe#getAndAddLong
public?final?long?getAndAddLong(Object?var1,?long?var2,?long?var4)?{
????long?var6;
????do?{
????????var6?=?this.getLongVolatile(var1,?var2);
????}?while(!this.compareAndSwapLong(var1,?var2,?var6,?var6?+?var4));

????return?var6;
}

底層使用的是CAS算法，JVM中的CAS操作是利用了處理器提供的CMPXCHG指令實(shí)現(xiàn)的。自旋CAS實(shí)現(xiàn)的基本思路就是循環(huán)進(jìn)行CAS操作直到成功為止，也正是因?yàn)檫@樣的實(shí)現(xiàn)思路也帶來了在高并發(fā)下的性能問題。循環(huán)時(shí)間長(zhǎng)開銷大，自旋CAS如果長(zhǎng)時(shí)間不成功，會(huì)給處理器帶來非常大的執(zhí)行開銷。在高并發(fā)環(huán)境下，N個(gè)線程同時(shí)進(jìn)行自旋操作，會(huì)出現(xiàn)大量失敗并不斷自旋的情況，所以在上述測(cè)試中，當(dāng)測(cè)試線程數(shù)非常多時(shí)，使用LongAdder的性能優(yōu)于使用AtomicLong。

LongAdder#increment方法分析

public?void?increment()?{
????add(1L);
}

public?void?add(long?x)?{
????Cell[]?as;?long?b,?v;?int?m;?Cell?a;
????if?((as?=?cells)?!=?null?||?!casBase(b?=?base,?b?+?x))?{
????????boolean?uncontended?=?true;
????????if?(as?==?null?||?(m?=?as.length?-?1)?0?||
????????????(a?=?as[getProbe()?&?m])?==?null?||
????????????!(uncontended?=?a.cas(v?=?a.value,?v?+?x)))
????????????longAccumulate(x,?null,?uncontended);
????}
}

final?void?longAccumulate(long?x,?LongBinaryOperator?fn,
??????????????????????????????boolean?wasUncontended)?{
????int?h;
????if?((h?=?getProbe())?==?0)?{
????????ThreadLocalRandom.current();?//?force?initialization
????????h?=?getProbe();
????????wasUncontended?=?true;
????}
????boolean?collide?=?false;????????????????//?True?if?last?slot?nonempty
????for?(;;)?{
????????Cell[]?as;?Cell?a;?int?n;?long?v;
????????if?((as?=?cells)?!=?null?&&?(n?=?as.length)?>?0)?{
????????????if?((a?=?as[(n?-?1)?&?h])?==?null)?{
????????????????if?(cellsBusy?==?0)?{???????//?Try?to?attach?new?Cell
????????????????????Cell?r?=?new?Cell(x);???//?Optimistically?create
????????????????????if?(cellsBusy?==?0?&&?casCellsBusy())?{
????????????????????????boolean?created?=?false;
????????????????????????try?{???????????????//?Recheck?under?lock
????????????????????????????Cell[]?rs;?int?m,?j;
????????????????????????????if?((rs?=?cells)?!=?null?&&
????????????????????????????????(m?=?rs.length)?>?0?&&
????????????????????????????????rs[j?=?(m?-?1)?&?h]?==?null)?{
????????????????????????????????rs[j]?=?r;
????????????????????????????????created?=?true;
????????????????????????????}
????????????????????????}?finally?{
????????????????????????????cellsBusy?=?0;
????????????????????????}
????????????????????????if?(created)
????????????????????????????break;
????????????????????????continue;???????????//?Slot?is?now?non-empty
????????????????????}
????????????????}
????????????????collide?=?false;
????????????}
????????????else?if?(!wasUncontended)???????//?CAS?already?known?to?fail
????????????????wasUncontended?=?true;??????//?Continue?after?rehash
????????????else?if?(a.cas(v?=?a.value,?((fn?==?null)???v?+?x?:
?????????????????????????????????????????????fn.applyAsLong(v,?x))))
????????????????break;
????????????else?if?(n?>=?NCPU?||?cells?!=?as)
????????????????collide?=?false;????????????//?At?max?size?or?stale
????????????else?if?(!collide)
????????????????collide?=?true;
????????????else?if?(cellsBusy?==?0?&&?casCellsBusy())?{
????????????????try?{
????????????????????if?(cells?==?as)?{??????//?Expand?table?unless?stale
????????????????????????Cell[]?rs?=?new?Cell[n?<1];
????????????????????????for?(int?i?=?0;?i?????????????????????????????rs[i]?=?as[i];
????????????????????????cells?=?rs;
????????????????????}
????????????????}?finally?{
????????????????????cellsBusy?=?0;
????????????????}
????????????????collide?=?false;
????????????????continue;???????????????????//?Retry?with?expanded?table
????????????}
????????????h?=?advanceProbe(h);
????????}
????????else?if?(cellsBusy?==?0?&&?cells?==?as?&&?casCellsBusy())?{
????????????boolean?init?=?false;
????????????try?{???????????????????????????//?Initialize?table
????????????????if?(cells?==?as)?{
????????????????????Cell[]?rs?=?new?Cell[2];
????????????????????rs[h?&?1]?=?new?Cell(x);
????????????????????cells?=?rs;
????????????????????init?=?true;
????????????????}
????????????}?finally?{
????????????????cellsBusy?=?0;
????????????}
????????????if?(init)
????????????????break;
????????}
????????else?if?(casBase(v?=?base,?((fn?==?null)???v?+?x?:
????????????????????????????????????????fn.applyAsLong(v,?x))))
????????????break;??????????????????????????//?Fall?back?on?using?base
????}
}
代碼很長(zhǎng)，可以結(jié)合圖片理解：

LongAdder性能高的原因是通過使用Cell數(shù)組，以空間換效率避免共享變量的競(jìng)爭(zhēng)，在LongAdder中內(nèi)部使用base變量保存Long值 ，當(dāng)沒有線程沖突時(shí)，使用CAS更新base的值，而存在線程沖突時(shí)，沒有執(zhí)行CAS成功的線程將CAS操作Cell數(shù)組，將數(shù)組中的元素置為1，即cell[i]=1，最后獲取計(jì)數(shù)時(shí)會(huì)計(jì)算cell[i]的總和在加base，即為最后的計(jì)數(shù)結(jié)果，sum代碼如下：

public?long?sum()?{
????Cell[]?as?=?cells;?Cell?a;
????long?sum?=?base;
????if?(as?!=?null)?{
????????for?(int?i?=?0;?i?????????????if?((a?=?as[i])?!=?null)
????????????????sum?+=?a.value;
????????}
????}
????return?sum;
}

| AtomicLong和LongAdder選擇

-End-
最近有一些小伙伴，讓我?guī)兔φ乙恍?面試題?資料，于是我翻遍了收藏的 5T 資料后，匯總整理出來，可以說是程序員面試必備！所有資料都整理到網(wǎng)盤了，歡迎下載！

點(diǎn)擊??卡片，關(guān)注后回復(fù)【面試題】即可獲取