网站建设的功能都需要有哪些,在线平台,哈尔滨品牌设计,项目可行性研究报告在JVM常用概念之新对象实例化博客中我讲到了对象的实例化#xff0c;主要包含分配#xff08;TLAB#xff09;、系统初始化、用户初始化#xff0c;而我在JVM常用概念之线程本地分配缓冲区#xff08;ThreadLocal Allocation Buffer#xff0c;TLAB#xff09;博客中也讲…在JVM常用概念之新对象实例化博客中我讲到了对象的实例化主要包含分配TLAB、系统初始化、用户初始化而我在JVM常用概念之线程本地分配缓冲区ThreadLocal Allocation BufferTLAB博客中也讲到TLAB分配的效率是非常高的而系统初始化和用户初始化是可以进行合并的那最后就剩下将数据写入内存这部分的成本没有讨论过那对于对象初始化而言写入内存这部分的成本是怎么样的呢?我们接下来通过实例讨论一下。
实验
源码
import org.openjdk.jmh.annotations.*;Warmup(iterations 5, time 1, timeUnit TimeUnit.SECONDS)
Measurement(iterations 5, time 1, timeUnit TimeUnit.SECONDS)
Fork(value 3)
BenchmarkMode(Mode.AverageTime)
OutputTimeUnit(TimeUnit.NANOSECONDS)
State(Scope.Benchmark)
public class Case {Param({1, 10, 100, 1000, 10000, 100000})int size;Benchmarkpublic byte[] java() {return new byte[size];}
}Score
Benchmark (size) Mode Cnt Score Error Units# Time to allocate
CASE.java 1 avgt 15 20.307 ± 4.532 ns/op
CASE.java 10 avgt 15 26.657 ± 6.072 ns/op
CASE.java 100 avgt 15 106.632 ± 34.742 ns/op
CASE.java 1000 avgt 15 681.176 ± 124.980 ns/op
CASE.java 10000 avgt 15 4576.433 ± 909.956 ns/op
CASE.java 100000 avgt 15 44881.095 ± 13765.440 ns/op# Allocation rate
CASE.java:·gc.alloc.rate 1 avgt 15 6228.153 ± 1059.385 MB/sec
CASE.java:·gc.alloc.rate 10 avgt 15 6335.809 ± 986.395 MB/sec
CASE.java:·gc.alloc.rate 100 avgt 15 6126.333 ± 1354.964 MB/sec
CASE.java:·gc.alloc.rate 1000 avgt 15 7772.263 ± 1263.453 MB/sec
CASE.java:·gc.alloc.rate 10000 avgt 15 11518.422 ± 2155.516 MB/sec
CASE.java:·gc.alloc.rate 100000 avgt 15 12039.594 ± 2724.242 MB/sec内存分配率直接由所运行机器的内存带宽/分配率决定。
热代码-汇编 0x00007f1f094f650b: movq $0x1,(%rdx) ; store mark word0.00% 0x00007f1f094f6512: prefetchnta 0xc0(%r9)0.64% 0x00007f1f094f651a: movl $0xf80000f5,0x8(%rdx) ; store klass word0.02% 0x00007f1f094f6521: mov %r11d,0xc(%rdx) ; store array length0x00007f1f094f6525: prefetchnta 0x100(%r9)0.05% 0x00007f1f094f652d: prefetchnta 0x140(%r9)0.07% 0x00007f1f094f6535: prefetchnta 0x180(%r9)0.09% 0x00007f1f094f653d: shr $0x3,%rcx0.00% 0x00007f1f094f6541: add $0xfffffffffffffffe,%rcx0x00007f1f094f6545: xor %rax,%rax0x00007f1f094f6548: cmp $0x8,%rcx╭ 0x00007f1f094f654c: jg 0x00007f1f094f655e ; large enough? jump│ 0x00007f1f094f654e: dec %rcx│╭ 0x00007f1f094f6551: js 0x00007f1f094f6565 ; zero length? jump││↗ 0x00007f1f094f6553: mov %rax,(%rdi,%rcx,8) ; small loop init│││ 0x00007f1f094f6557: dec %rcx││╰ 0x00007f1f094f655a: jge 0x00007f1f094f6553││ ╭ 0x00007f1f094f655c: jmp 0x00007f1f094f6565↘│ │ 0x00007f1f094f655e: shl $0x3,%rcx89.12% │ │ 0x00007f1f094f6562: rep rex.W stos %al,%es:(%rdi) ; large loop init0.20% ↘ ↘ 0x00007f1f094f6565: mov %r8,(%rsp)可发现大初始化循环的内联的rep stos序列占用了大部分的时间成本。
思考
可以创建一个分配未初始化的对象吗?
实际应用场景下创建一个分配未初始化的对象没有实际意义而通过Unsafe可以创建分配未初始化的对象Unsafe不遵循Java规范有时候甚至违反JVM规范可以通过jdk.internal.*使用它但使用Unsafe可能导致JVM崩溃等未知的问题及风险请谨慎使用
源码
import jdk.internal.misc.Unsafe;
import org.openjdk.jmh.annotations.*;Warmup(iterations 5, time 1, timeUnit TimeUnit.SECONDS)
Measurement(iterations 5, time 1, timeUnit TimeUnit.SECONDS)
Fork(value 3)
BenchmarkMode(Mode.AverageTime)
OutputTimeUnit(TimeUnit.NANOSECONDS)
State(Scope.Benchmark)
public class Case {static Unsafe U;static {try {Field field Unsafe.class.getDeclaredField(theUnsafe);field.setAccessible(true);U (Unsafe) field.get(null);} catch (Exception e) {throw new IllegalStateException(e);}}Param({1, 10, 100, 1000, 10000, 100000})int size;Benchmarkpublic byte[] unsafe() {return (byte[]) U.allocateUninitializedArray(byte.class, size);}
}Score
Benchmark (size) Mode Cnt Score Error Units
Case.unsafe 1 avgt 15 19.766 ± 4.002 ns/op
Case.unsafe 10 avgt 15 27.486 ± 7.005 ns/op
Case.unsafe 100 avgt 15 80.040 ± 15.754 ns/op
Case.unsafe 1000 avgt 15 156.041 ± 0.552 ns/op
Case.unsafe 10000 avgt 15 162.384 ± 1.448 ns/op
Case.unsafe 100000 avgt 15 309.769 ± 2.819 ns/opCase.unsafe:·gc.alloc.rate 1 avgt 15 6359.987 ± 928.472 MB/sec
Case.unsafe:·gc.alloc.rate 10 avgt 15 6193.103 ± 1160.353 MB/sec
Case.unsafe:·gc.alloc.rate 100 avgt 15 7855.147 ± 1313.314 MB/sec
Case.unsafe:·gc.alloc.rate 1000 avgt 15 33171.384 ± 153.645 MB/sec
Case.unsafe:·gc.alloc.rate 10000 avgt 15 315740.299 ± 3678.459 MB/sec
Case.unsafe:·gc.alloc.rate 100000 avgt 15 1650860.763 ± 14498.920 MB/sec热代码-汇编 0x00007f65fd722c74: prefetchnta 0xc0(%r11)66.06% 0x00007f65fd722c7c: movq $0x1,(%rax) ; store mark word0.40% 0x00007f65fd722c83: prefetchnta 0x100(%r11)4.43% 0x00007f65fd722c8b: movl $0xf80000f5,0x8(%rax) ; store class word0.01% 0x00007f65fd722c92: mov %edx,0xc(%rax) ; store array length0x00007f65fd722c95: prefetchnta 0x140(%r11)5.18% 0x00007f65fd722c9d: prefetchnta 0x180(%r11)4.99% 0x00007f65fd722ca5: mov %r8,0x40(%rsp)0x00007f65fd722caa: mov %rax,%rdx将元数据写入内存占用了大部分时间成本。
