当goroutine争用超过3400时，为什么sync.Mutex会大大降低性能？

我正在比较关于sync.Mutex和Go频道的性能。这是我的基准：

// go playground: https://play.golang.org/p/f_u9jHBq_Jc
const (
    start = 300  // actual = start  * goprocs
    end   = 600 // actual = end    * goprocs
    step  = 10
)

var goprocs = runtime.GOMAXPROCS(0) // 8

// https://perf.golang.org/search?q=upload:20190819.3
func BenchmarkChanWrite(b *testing.B) {
    var v int64
    ch := make(chan int, 1)
    ch <- 1
    for i := start; i 

性能比较可视化如下：



是什么原因 


当goroutine的数量超过大约3400时，sync.Mutex的性能下降很大吗？
Go频道非常稳定，但比同步慢。


Benchstat的原始工作台数据（进行测试-bench =。-count = 5）go version go1.12.4 linux/amd64：

MutexWrite/goroutines-2400-8  48.6ns ± 1%
MutexWrite/goroutines-2480-8  49.1ns ± 0%
MutexWrite/goroutines-2560-8  49.7ns ± 1%
MutexWrite/goroutines-2640-8  50.5ns ± 3%
MutexWrite/goroutines-2720-8  50.9ns ± 2%
MutexWrite/goroutines-2800-8  51.8ns ± 3%
MutexWrite/goroutines-2880-8  52.5ns ± 2%
MutexWrite/goroutines-2960-8  54.1ns ± 4%
MutexWrite/goroutines-3040-8  54.5ns ± 2%
MutexWrite/goroutines-3120-8  56.1ns ± 3%
MutexWrite/goroutines-3200-8  63.2ns ± 5%
MutexWrite/goroutines-3280-8  77.5ns ± 6%
MutexWrite/goroutines-3360-8   141ns ± 6%
MutexWrite/goroutines-3440-8   239ns ± 8%
MutexWrite/goroutines-3520-8   248ns ± 3%
MutexWrite/goroutines-3600-8   254ns ± 2%
MutexWrite/goroutines-3680-8   256ns ± 1%
MutexWrite/goroutines-3760-8   261ns ± 2%
MutexWrite/goroutines-3840-8   266ns ± 3%
MutexWrite/goroutines-3920-8   276ns ± 3%
MutexWrite/goroutines-4000-8   278ns ± 3%
MutexWrite/goroutines-4080-8   286ns ± 5%
MutexWrite/goroutines-4160-8   293ns ± 4%
MutexWrite/goroutines-4240-8   295ns ± 2%
MutexWrite/goroutines-4320-8   280ns ± 8%
MutexWrite/goroutines-4400-8   294ns ± 9%
MutexWrite/goroutines-4480-8   285ns ±10%
MutexWrite/goroutines-4560-8   290ns ± 8%
MutexWrite/goroutines-4640-8   271ns ± 3%
MutexWrite/goroutines-4720-8   271ns ± 4%

ChanWrite/goroutines-2400-8  158ns ± 3%
ChanWrite/goroutines-2480-8  159ns ± 2%
ChanWrite/goroutines-2560-8  161ns ± 2%
ChanWrite/goroutines-2640-8  161ns ± 1%
ChanWrite/goroutines-2720-8  163ns ± 1%
ChanWrite/goroutines-2800-8  166ns ± 3%
ChanWrite/goroutines-2880-8  168ns ± 1%
ChanWrite/goroutines-2960-8  176ns ± 4%
ChanWrite/goroutines-3040-8  176ns ± 2%
ChanWrite/goroutines-3120-8  180ns ± 1%
ChanWrite/goroutines-3200-8  180ns ± 1%
ChanWrite/goroutines-3280-8  181ns ± 2%
ChanWrite/goroutines-3360-8  183ns ± 2%
ChanWrite/goroutines-3440-8  188ns ± 3%
ChanWrite/goroutines-3520-8  190ns ± 2%
ChanWrite/goroutines-3600-8  193ns ± 2%
ChanWrite/goroutines-3680-8  196ns ± 3%
ChanWrite/goroutines-3760-8  199ns ± 2%
ChanWrite/goroutines-3840-8  206ns ± 2%
ChanWrite/goroutines-3920-8  209ns ± 2%
ChanWrite/goroutines-4000-8  206ns ± 2%
ChanWrite/goroutines-4080-8  209ns ± 2%
ChanWrite/goroutines-4160-8  208ns ± 2%
ChanWrite/goroutines-4240-8  209ns ± 3%
ChanWrite/goroutines-4320-8  213ns ± 2%
ChanWrite/goroutines-4400-8  209ns ± 2%
ChanWrite/goroutines-4480-8  211ns ± 1%
ChanWrite/goroutines-4560-8  213ns ± 2%
ChanWrite/goroutines-4640-8  215ns ± 1%
ChanWrite/goroutines-4720-8  218ns ± 3%


转到1.12.4。硬件：

CPU:       Quad core Intel Core i7-7700 (-MT-MCP-) cache: 8192 KB
           clock speeds: max: 4200 MHz 1: 1109 MHz 2: 3641 MHz 3: 3472 MHz 4: 3514 MHz 5: 3873 MHz 6: 3537 MHz
           7: 3410 MHz 8: 3016 MHz
           CPU Flags: 3dnowprefetch abm acpi adx aes aperfmperf apic arat arch_perfmon art avx avx2 bmi1 bmi2
           bts clflush clflushopt cmov constant_tsc cpuid cpuid_fault cx16 cx8 de ds_cpl dtes64 dtherm dts epb
           ept erms est f16c flexpriority flush_l1d fma fpu fsgsbase fxsr hle ht hwp hwp_act_window hwp_epp
           hwp_notify ibpb ibrs ida intel_pt invpcid invpcid_single lahf_lm lm mca mce md_clear mmx monitor
           movbe mpx msr mtrr nonstop_tsc nopl nx pae pat pbe pcid pclmulqdq pdcm pdpe1gb pebs pge pln pni
           popcnt pse pse36 pti pts rdrand rdseed rdtscp rep_good rtm sdbg sep smap smep smx ss ssbd sse sse2
           sse4_1 sse4_2 ssse3 stibp syscall tm tm2 tpr_shadow tsc tsc_adjust tsc_deadline_timer tsc_known_freq
           vme vmx vnmi vpid x2apic xgetbv1 xsave xsavec xsaveopt xsaves xtopology xtpr




更新：我在不同的硬件上进行了测试。看来问题仍然存在：



板凳：https : //play.golang.org/p/HnQ44--E4UQ



更新：

我的完整基准测试从8个goroutine到15000个goroutine，包括chan / sync.Mutex / atomic的比较：