runtime: document that MemStats.Lookups is always zero

[zc24]

package main
import "fmt"
import "runtime"
func main () {
var a = runtime.MemStats{}
runtime.ReadMemStats(&a)
fmt.Printf("%+v\n", a.Lookups)
}

[dmitshur]

Please include answers to questions from the issue template:

• What did you do?
• What did you expect to see?
• What operating system and processor architecture are you using (go env)?
• Does this issue reproduce with the latest release?

Note that the release policy means there will be minor releases for Go 1.19 and 1.18, not older.

[zc24]

@dmitshur hello，I want to get the value of runtime.MEMstats.Lookups in the latest version 1.19, but It's always going to be 0. I see that in the 1.11 submission history, someone changed this, and in the end, lookups was discarded, but the reason is not explained clearly.I want to know the reason for the abandonment

[go101]

Yes, 1.10 prints 3 (on linux/amd64). But many runtime internal implementation details change from version to version. So this is not a sufficient evidence to prove the field is discarded.

[zc24]

@go101 @dmitshur Which method will trigger the increase of Lookups in the latest version 1.19.I don't seem to be able to find anywhere that would modify the value of lookups effectively

[go101]

It does looks the Lookups field is never used as L-values now.

[ZekeLu]

I think this is changed by 41e6abd. The comment for the next field was changed in that commit:

go/src/runtime/mstats.go

Line 29 in 41e6abd

nlookup uint64 // number of pointer lookups (unused)

-   nlookup     uint64 // number of pointer lookups
+   nlookup     uint64 // number of pointer lookups (unused)

[zc24]

@ZekeLu I looked at the submission information, but I couldn't see the fundamental reason why the lookups count was dropped, perhaps because the statistic itself doesn't mean much.The field is still there for downward compatibility

[dmitshur]

@mknyszek Is this something you're familiar with? The issue here seems to be that the MemStats.Lookups is never populated on tip, and its internal comment on mstats.nlookup says it's unused. However, the publicly visible comment doesn't indicate that it's deprecated or unused, just that it's "useful for debugging runtime internals".

Does the documentation need to be updated, or is its current description fine and this is working as intended?

[prattmic]

I believe this just needs a documentation update to say the field is deprecated.

[mknyszek]

What Michael said is correct, the documentation is just out of date. As for why it was set to zero, one reason why might be that this needs to be tracked on a very hot path in the runtime, so it's a cost everyone would pay for.

I'll update the docs.