Rıf*_*hin 3 c# memory performance
为什么拳击和拆箱操作之间的速度变化如此之大？有10倍的差异.我们什么时候应该关心这个？上周Azure支持告诉我们,我们的应用程序的堆内存中存在问题.我很想知道它是否与装箱拆箱问题有关.
using System;
using System.Diagnostics;
namespace ConsoleBoxing
{
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Program started");
var elapsed = Boxing();
Unboxing(elapsed);
Console.WriteLine("Program ended");
Console.Read();
}
private static void Unboxing(double boxingtime)
{
Stopwatch s = new Stopwatch();
s.Start();
for (int i = 0; i < 1000000; i++)
{
int a = 33;//DATA GOES TO STACK
object b = a;//HEAP IS REFERENCED
int c = (int)b;//unboxing only hEre ....HEAP GOES TO STACK
}
s.Stop();
var UnBoxing = s.Elapsed.TotalMilliseconds- boxingtime;
Console.WriteLine("UnBoxing time : " + UnBoxing);
}
private static double Boxing()
{
Stopwatch s = new Stopwatch();
s.Start();
for (int i = 0; i < 1000000; i++)
{
int a = 33;
object b = a;
}
s.Stop();
var elapsed = s.Elapsed.TotalMilliseconds;
Console.WriteLine("Boxing time : " + elapsed);
return elapsed;
}
}
}
尽管人们已经为为什么拆箱比装箱更快提供了很好的解释。我想多说一点你用来测试性能差异的方法。
您是否从您发布的代码中得到了结果（速度相差 10 倍）？如果我在发布模式下运行该程序，则输出如下：
Program started
Boxing time : 0.2741
UnBoxing time : 4.5847
Program ended
每当我进行微观性能基准测试时，我倾向于进一步验证我确实在比较我打算比较的操作。编译器可以对您的代码进行优化。在 ILDASM 中打开可执行文件：
这是取消装箱的 IL：（我只包括了最重要的部分）
IL_0000: newobj instance void [System]System.Diagnostics.Stopwatch::.ctor()
IL_0005: stloc.0
IL_0006: ldloc.0
IL_0007: callvirt instance void [System]System.Diagnostics.Stopwatch::Start()
IL_000c: ldc.i4.0
IL_000d: stloc.1
IL_000e: br.s IL_0025
IL_0010: ldc.i4.s 33
IL_0012: stloc.2
IL_0013: ldloc.2
IL_0014: box [mscorlib]System.Int32 //Here is the boxing
IL_0019: stloc.3
IL_001a: ldloc.3
IL_001b: unbox.any [mscorlib]System.Int32 //Here is the unboxing
IL_0020: pop
IL_0021: ldloc.1
IL_0022: ldc.i4.1
IL_0023: add
IL_0024: stloc.1
IL_0025: ldloc.1
IL_0026: ldc.i4 0xf4240
IL_002b: blt.s IL_0010
IL_002d: ldloc.0
IL_002e: callvirt instance void [System]System.Diagnostics.Stopwatch::Stop()
这是拳击的代码：
IL_0000: newobj instance void [System]System.Diagnostics.Stopwatch::.ctor()
IL_0005: stloc.0
IL_0006: ldloc.0
IL_0007: callvirt instance void [System]System.Diagnostics.Stopwatch::Start()
IL_000c: ldc.i4.0
IL_000d: stloc.1
IL_000e: br.s IL_0017
IL_0010: ldc.i4.s 33
IL_0012: stloc.2
IL_0013: ldloc.1
IL_0014: ldc.i4.1
IL_0015: add
IL_0016: stloc.1
IL_0017: ldloc.1
IL_0018: ldc.i4 0xf4240
IL_001d: blt.s IL_0010
IL_001f: ldloc.0
IL_0020: callvirt instance void [System]System.Diagnostics.Stopwatch::Stop()
拳击方法中根本没有拳击指令。它已被编译器完全删除。Boxing 方法只是迭代一个空循环。因此，在拆箱中测量的时间成为装箱和拆箱的总时间。
微基准测试很容易受到编译器技巧的影响。我建议你也看看你的IL。如果您使用不同的编译器，它可能会有所不同。
我稍微修改了你的测试代码：
拳法：
private static object Boxing()
{
Stopwatch s = new Stopwatch();
int unboxed = 33;
object boxed = null;
s.Start();
for (int i = 0; i < 1000000; i++)
{
boxed = unboxed;
}
s.Stop();
var elapsed = s.Elapsed.TotalMilliseconds;
Console.WriteLine("Boxing time : " + elapsed);
return boxed;
}
和开箱方法：
private static int Unboxing()
{
Stopwatch s = new Stopwatch();
object boxed = 33;
int unboxed = 0;
s.Start();
for (int i = 0; i < 1000000; i++)
{
unboxed = (int)boxed;
}
s.Stop();
var time = s.Elapsed.TotalMilliseconds;
Console.WriteLine("UnBoxing time : " + time);
return unboxed;
}
以便将它们翻译成类似的 IL：
对于拳击方法：
IL_000c: callvirt instance void [System]System.Diagnostics.Stopwatch::Start()
IL_0011: ldc.i4.0
IL_0012: stloc.3
IL_0013: br.s IL_0020
IL_0015: ldloc.1
IL_0016: box [mscorlib]System.Int32 //Here is the boxing
IL_001b: stloc.2
IL_001c: ldloc.3
IL_001d: ldc.i4.1
IL_001e: add
IL_001f: stloc.3
IL_0020: ldloc.3
IL_0021: ldc.i4 0xf4240
IL_0026: blt.s IL_0015
IL_0028: ldloc.0
IL_0029: callvirt instance void [System]System.Diagnostics.Stopwatch::Stop()
对于拆箱：
IL_0011: callvirt instance void [System]System.Diagnostics.Stopwatch::Start()
IL_0016: ldc.i4.0
IL_0017: stloc.3
IL_0018: br.s IL_0025
IL_001a: ldloc.1
IL_001b: unbox.any [mscorlib]System.Int32 //Here is the UnBoxng
IL_0020: stloc.2
IL_0021: ldloc.3
IL_0022: ldc.i4.1
IL_0023: add
IL_0024: stloc.3
IL_0025: ldloc.3
IL_0026: ldc.i4 0xf4240
IL_002b: blt.s IL_001a
IL_002d: ldloc.0
IL_002e: callvirt instance void [System]System.Diagnostics.Stopwatch::Stop()
运行几个循环来消除冷启动效应：
static void Main(string[] args)
{
Console.WriteLine("Program started");
for (int i = 0; i < 10; i++)
{
Boxing();
Unboxing();
}
Console.WriteLine("Program ended");
Console.Read();
}
这是输出：
Program started
Boxing time : 3.4814
UnBoxing time : 0.1712
Boxing time : 2.6294
...
Boxing time : 2.4842
UnBoxing time : 0.1712
Program ended
这是否证明拆箱比装箱快10 倍？让我们用windbg检查汇编代码：
0:004> !u 000007fe93b83940
Normal JIT generated code
MicroBenchmarks.Program.Boxing()
...
000007fe`93ca01b3 call System_ni+0x2905e0 (000007fe`f07a05e0) (System.Diagnostics.Stopwatch.GetTimestamp(), mdToken: 00000000060040d2)
...
//This is the for loop
000007fe`93ca01c2 mov eax,21h
000007fe`93ca01c7 mov dword ptr [rsp+20h],eax
000007fe`93ca01cb lea rdx,[rsp+20h]
000007fe`93ca01d0 lea rcx,[mscorlib_ni+0x6e92b0 (000007fe`f18b92b0)]
//here is the boxing
000007fe`93ca01d7 call clr!JIT_BoxFastMP_InlineGetThread (000007fe`f33126d0)
000007fe`93ca01dc mov rsi,rax
//loop unrolling. instead of increment i by 1, we are actually incrementing i by 4
000007fe`93ca01df add edi,4
000007fe`93ca01e2 cmp edi,0F4240h // 0F4240h = 1000000
000007fe`93ca01e8 jl 000007fe`93ca01c2 // jumps to the line "mov eax,21h"
//end of the for loop
000007fe`93ca01ea mov rcx,rbx
000007fe`93ca01ed call System_ni+0x2acb70 (000007fe`f07bcb70) (System.Diagnostics.Stopwatch.Stop(), mdToken: 00000000060040cb)
拆箱组装：
0:004> !u 000007fe93b83930
Normal JIT generated code
MicroBenchmarks.Program.Unboxing()
Begin 000007fe93ca02c0, size 117
000007fe`93ca02c0 push rbx
...
000007fe`93ca030a call System_ni+0x2905e0 (000007fe`f07a05e0) (System.Diagnostics.Stopwatch.GetTimestamp(), mdToken: 00000000060040d2)
000007fe`93ca030f mov qword ptr [rbx+10h],rax
000007fe`93ca0313 mov byte ptr [rbx+18h],1
000007fe`93ca0317 xor eax,eax
000007fe`93ca0319 mov edi,dword ptr [rdi+8]
000007fe`93ca031c nop dword ptr [rax]
//This is the for loop
//again, loop unrolling
000007fe`93ca0320 add eax,4
000007fe`93ca0323 cmp eax,0F4240h // 0F4240h = 1000000
000007fe`93ca0328 jl 000007fe`93ca0320 //jumps to "add eax,4"
//end of the for loop
000007fe`93ca032a mov rcx,rbx
000007fe`93ca032d call System_ni+0x2acb70 (000007fe`f07bcb70) (System.Diagnostics.Stopwatch.Stop(), mdToken: 00000000060040cb)
您可以看到，即使在 IL 级别比较似乎是合理的，JIT 仍然可以在运行时执行另一个优化。UnBoxing 方法再次执行空循环。在您验证为两种方法执行的代码具有可比性之前，很难简单地得出“拆箱比装箱快 10 倍”的结论
将拆箱视为从盒装对象到寄存器的单个内存加载指令.可能有一些周围的地址计算和转换验证逻辑.盒装对象就像一个带有一个盒装类型字段的类.这些操作有多贵？不是很特别,因为基准测试中的L1缓存命中率约为100%.
拳击涉及分配一个新的对象和GC以后.在您的代码中,GC可能会在99%的情况下触发分配.
这表示你的基准测试无效,因为循环没有副作用.目前的JIT可能无法优化它们.以某种方式让循环计算结果并将其汇入其中GC.KeepAlive以使结果显示为使用.此外,您可能正在运行调试模式.