CompareExchange_long benchmark sometimes reports very long execution time on x86 #1497

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1. AndyAyersMS commented on Oct 27, 2020

   

   AndyAyersMS

   on Oct 27, 2020

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   Here's a repro... issue is lock cmpxchg8 splitting a cache line, so it requires a particular data alignment.

   The code below has a 50/50 chance that one of those long fields does this.

   public class X
   {
        int x;
        long l1;
        long l2;
        long l3;
        long l4;
        long l5;
        long l6;
        long l7;
        long l8;
        
        [Benchmark]
        public long CompareExchange_long1() => Interlocked.CompareExchange(ref l1, 1, 0);
        
        [Benchmark]
        public long CompareExchange_long2() => Interlocked.CompareExchange(ref l2, 1, 0);
        
        [Benchmark]
        public long CompareExchange_long3() => Interlocked.CompareExchange(ref l3, 1, 0);
        
        [Benchmark]
        public long CompareExchange_long4() => Interlocked.CompareExchange(ref l4, 1, 0);
        
        [Benchmark]
        public long CompareExchange_long5() => Interlocked.CompareExchange(ref l5, 1, 0);
        
        [Benchmark]
        public long CompareExchange_long6() => Interlocked.CompareExchange(ref l6, 1, 0);
        
        [Benchmark]
        public long CompareExchange_long7() => Interlocked.CompareExchange(ref l7, 1, 0);
        
        [Benchmark]
        public long CompareExchange_long8() => Interlocked.CompareExchange(ref l8, 1, 0);
   }

   Sample data

   BenchmarkDotNet=v0.12.1, OS=Windows 10.0.19041.572 (2004/?/20H1)
   Intel Core i7-8650U CPU 1.90GHz (Kaby Lake R), 1 CPU, 8 logical and 4 physical cores
   .NET Core SDK=5.0.100-rc.2.20479.15
     [Host]     : .NET Core 5.0.0 (CoreCLR 5.0.20.47505, CoreFX 5.0.20.47505), X86 RyuJIT
     DefaultJob : .NET Core 5.0.0 (CoreCLR 5.0.20.47505, CoreFX 5.0.20.47505), X86 RyuJIT
   

   Method	Mean	Error	StdDev	Median
   CompareExchange_long1	9.083 ns	0.5248 ns	0.5615 ns	9.009 ns
   CompareExchange_long2	11.070 ns	0.4912 ns	0.4102 ns	11.115 ns
   CompareExchange_long3	11.995 ns	0.5210 ns	0.9785 ns	11.782 ns
   CompareExchange_long4	11.636 ns	0.5162 ns	0.6891 ns	11.585 ns
   CompareExchange_long5	1,273.176 ns	25.7278 ns	71.7188 ns	1,254.676 ns
   CompareExchange_long6	10.818 ns	0.7369 ns	2.1380 ns	9.710 ns
   CompareExchange_long7	9.389 ns	0.2530 ns	0.2243 ns	9.346 ns
   CompareExchange_long8	9.397 ns	0.3290 ns	0.3078 ns	9.449 ns

   👍React with 👍3Reacted by Dan Moseley, Kunal Pathak and Egor Bogatov
2. AndyAyersMS commented on Oct 27, 2020

   

   AndyAyersMS

   on Oct 27, 2020

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   Also explains why we don't see this on x64, because classes there are 8 byte aligned. On x86, classes are only 4 byte aligned.
3. danmoseley commented on Oct 27, 2020

   

   danmoseley

   on Oct 27, 2020

   Contributor

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   cc @kunalspathak