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

[Robert Engels]

Another method would be to change the operation under test, to touch and read a large memory block - turning off the timer while this happens - so that only the concurrency ops are timed - I think then you will no longer see the 'super high performance' with the low routine counts. That is, do 'more work' yet constant that destroys the cache - and back into the concurrency times.

-----Original Message-----
From: changkun
Sent: Aug 26, 2019 2:23 PM
To: golang-nuts
Subject: Re: [go-nuts] sync.Mutex encounter large performance drop when goroutine contention more than 3400

Your cache theory is very reasonable, but this was clear in the beginning post:  "before or after the massive increase, performance drops linearly".

Your hypothesis is reasonable, but how can you prove your hypothesis? By host machine cache usage monitoring? 

Matching of a concept is still not persuasive.

On Monday, August 26, 2019 at 8:08:27 PM UTC+2, Robert Engels wrote:

Which is what I would expect - once the number of routines exhaust the cache, it will take the next level (or never since its main memory) to see an massive increase in time. 4800 is 30% slower than 3600 - so it is increasing linearly with the number of Go routines.

-----Original Message-----
From: changkun
Sent: Aug 26, 2019 11:49 AM
To: golang-nuts
Subject: Re: [go-nuts] sync.Mutex encounter large performance drop when goroutine contention more than 3400

According to your formula let's sample three points:

2400 goroutines: 2.508s/(50000000*2400) = 2.09 × 10^-11 s

3600 goroutines: 12.219s/(50000000*3600) = 6.78833333 × 10-11 seconds

4800 goroutines: 16.020s/(50000000*4800) = 6.67500 × 10^-11 s

One can observe that 3600 and 4800 mostly equal to each other, but they both three times slower than 2400.

goos: linux

goarch: amd64

BenchmarkMutexWrite/goroutines-2400-8           50000000                46.5 ns/op

PASS

ok      _/home/changkun/dev/tests       2.508s

goos: linux

goarch: amd64

BenchmarkMutexWrite/goroutines-3600-8           50000000               240 ns/op

PASS

ok      _/home/changkun/dev/tests       12.219s

goos: linux

goarch: amd64

BenchmarkMutexWrite/goroutines-4800-8           50000000               317 ns/op

PASS

ok      _/home/changkun/dev/tests       16.020s

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