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runtime: probably unwanted integer division in scavenger pacing #44036

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dominikh opened this issue Feb 1, 2021 · 2 comments
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runtime: probably unwanted integer division in scavenger pacing #44036

dominikh opened this issue Feb 1, 2021 · 2 comments
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FrozenDueToAge NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
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Go1.17

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@dominikh
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dominikh commented Feb 1, 2021 •
edited

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.
@ALTree ALTree changed the title Probably unwanted integer division in scavenger pacing runtime: probably unwanted integer division in scavenger pacing Feb 1, 2021
@ALTree ALTree added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Feb 1, 2021
@mknyszek
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mknyszek commented Feb 1, 2021

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.

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

@mknyszek mknyszek self-assigned this Apr 23, 2021
@mknyszek mknyszek added this to the Go1.17 milestone Apr 23, 2021
@golang golang locked and limited conversation to collaborators Apr 26, 2022
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