runtime: probably unwanted integer division in scavenger pacing

[dominikh]

I've stumbled across the following code, which IMO is incorrect:

go/src/runtime/mgcscavenge.go

Lines 371 to 378 in 6ac91e4

	// Set a lower bound on the fraction.
	// Due to OS-related anomalies we may "sleep" for an inordinate amount
	// of time. Let's avoid letting the ratio get out of hand by bounding
	// the sleep time we use in our EWMA.
	const minFraction = 1 / 1000
	if fraction < minFraction {
	fraction = minFraction
	}

The value of minFraction is 0, which makes the check pointless.

/cc @mknyszek to confirm my suspicion, as I'm not familiar with any of this code.

[mknyszek]

Erg, yeah good catch, thank you. I like to think that constants are truly untyped, but that's wrong. :)

Luckily, it looks like we didn't hit this case much in practice or not any case that was important enough for someone to report a bug about it. This makes sense since the scavenger isn't quite as useful in the context of, say, a CLI tool, where it's more likely this case could happen (e.g. laptop lid closes during the section where we're measuring time). I'll send a fix, but it's probably not worth backporting.

[gopherbot]

Change https://golang.org/cl/313249 mentions this issue: runtime: fix scavenge min fraction constant floor division