Documentation: some math in the Garbage Collection guide is incorrect

[MikeMitchellWebDev]

Go version

1.22

Output of go env in your module/workspace:

n/a

What did you do?

I double checked the math in this blog post about the garbage collector and it seems incorrect. https://tip.golang.org/doc/gc-guide

Meanwhile, target heap memory is defined as:

Target heap memory = Live heap + (Live heap + GC roots) * GOGC / 100

As an example, consider a Go program with a live heap size of 8 MiB, 1 MiB of goroutine stacks, and 1 MiB of pointers in global variables. Then, with a GOGC value of 100, the amount of new memory that will be allocated before the next GC runs will be 10 MiB, or 100% of the 10 MiB of work, for a total heap footprint of 18 MiB. With a GOGC value of 50, then it'll be 50%, or 5 MiB. With a GOGC value of 200, it'll be 200%, or 20 MiB.

What did you see happen?

I see two out of three equations in the paragraph as being incorrect.

Correct

*8 + (8+1+1)100/100 = 18

**Incorrect

8 + (8+1+1)*200/100 = 20

8 + (8+1+1)50/100 = 5*

What did you expect to see?

I attached some screenshots from the online calculator. When GOGC is 50, with the given numbers, the expected total should be 13 MiB (not 5). When GOGC is 200, with the given numbers, the expected total should be 28 MiB (not 20).

[zigo101]

Those numbers are the amount of new memory, a.k.a. the (Live heap + GC roots) * GOGC / 100 part.

[MikeMitchellWebDev]

Those numbers are the amount of new memory, a.k.a. the (Live heap + GC roots) * GOGC / 100 part.

Your sentence ("those numbers") is a little bit vague. What numbers exactly? I ask that because you only quote part of the equation in question. Did you deliberately omit the first "Live heap" from the equation?

[ Target heap memory = Live heap + (Live heap + GC roots) * GOGC / 100]

Either way, I think I disagree with you, because the math is correct for one of the three equations.

Your remark seems to imply that I misunderstand the whole principle of the equation. However, if, as you seem to suggest, I misunderstood the terminology (and all the math was in fact correct), then how is it possible that I report one of the three equations as correct? Can you clarify?

[zigo101]

It clearly says that 18 is total and others are new.

[cherrymui]

The three numbers are

Then, with a GOGC value of 100, the amount of new memory that will be allocated before the next GC runs will be 10 MiB
With a GOGC value of 50, then it'll be 50%, or 5 MiB.
With a GOGC value of 200, it'll be 200%, or 20 MiB.

which match your numbers minus 8.

I don't see an issue here. Closing. Thanks.