Re: [go-nuts] sync.Mutex encounter large performance drop when goroutine contention more than 3400

[Robert Engels]

I think you'll find the reason that the Mutex uses the Go scheduler. The chan is controlled by a 'mutex' which eventually defers to the OS futex - and the OS futex is probably more efficient at scheduling in the face of large contention - although you would think it should be the other way around.

I am guessing that if you logged the number of operations done per Go routine, you will see that the Mutex version is very fair, and the chan/futex version is unfair - meaning many are starved.

-----Original Message-----
From: changkun
Sent: Aug 19, 2019 12:50 PM
To: golang-nuts
Subject: [go-nuts] sync.Mutex encounter large performance drop when goroutine contention more than 3400

I am comparing the performance regarding sync.Mutex and Go channels. Here is my benchmark: https://play.golang.org/p/zLjVtsSx9gd

The performance comparison visualization is as follows:

What are the reasons that 

1. sync.Mutex encounter a large performance drop when the number of goroutines goes higher than roughly 3400?

2. Go channels are pretty stable but slower than sync.Mutex before?

Raw bench data by benchstat (go test -bench=. -count=5):

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%

--
You received this message because you are subscribed to the Google Groups "golang-nuts" group.
To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/3275fb21-dfbd-411d-be42-683386e7ebe2%40googlegroups.com.

-- 

You received this message because you are subscribed to the Google Groups "golang-nuts" group.

To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com.

To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/372639329.9450.1566246407416%40wamui-abby.atl.sa.earthlink.net.