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testing: Benchmark shows zero allocations #20863

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testing: Benchmark shows zero allocations#20863
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DocumentationIssues describing a change to documentation.FrozenDueToAgeNeedsDecisionFeedback is required from experts, contributors, and/or the community before a change can be made.
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Go1.9

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@rogpeppe
rogpeppe
opened on Jun 30, 2017

testing.Benchmark does not seem to produce valid memory stats any more:

The following program:

package main

import (
	"fmt"
	"testing"
)

var global *int

func main() {
	r := testing.Benchmark(func(b *testing.B) {
		for i := 0; i < b.N; i++ {
			global = new(int)
		}
	})
	fmt.Printf("creating filter: %v\n", r.MemString())
}

always prints:

       0 B/op	       0 allocs/op

I'd expect it to print

       8 B/op	       1 allocs/op

like previous Go versions.

go version devel +eab99a8 Mon Jun 26 21:12:22 2017 +0000 linux/amd64

Activity

  1. rogpeppe
    changed the title [-]testing: testing.Benchmark shows zero allocations[/-] [+]testing: Benchmark shows zero allocations[/+] on Jun 30, 2017

  2. rogpeppe commented on Jun 30, 2017

    @rogpeppe
    rogpeppe
    on Jun 30, 2017
    ContributorAuthor

    It turns out that it prints memory statistics when b.ReportAllocs is called.
    As this is a behaviour change, perhaps this should be documented in
    the Go 1.9 release notes.

  3. ALTree commented on Jun 30, 2017

    @ALTree
    ALTree
    on Jun 30, 2017
    Member

    This is already documented

    MemAllocs and MemBytes may be zero if memory benchmarking is not requested using B.ReportAllocs or the -benchmem command line flag.

    http://tip.golang.org/pkg/testing/#BenchmarkResult

    but if it was changed in go1.9 then I agree it's worth mentioning in release notes, too.

  4. ALTree
    added
    DocumentationIssues describing a change to documentation.
    on Jun 30, 2017
  5. ALTree
    added this to the Go1.9 milestone on Jun 30, 2017
  6. ALTree
    added
    NeedsDecisionFeedback is required from experts, contributors, and/or the community before a change can be made.
    on Jun 30, 2017

  7. ALTree commented on Jun 30, 2017

    @ALTree
    ALTree
    on Jun 30, 2017
    Member

    The change was introduced in https://go-review.googlesource.com/c/36791/

  8. gopherbot commented on Jun 30, 2017

    @gopherbot
    gopherbot
    on Jun 30, 2017
    Contributor

    CL https://golang.org/cl/47331 mentions this issue.

  9. bradfitz commented on Jun 30, 2017

    @bradfitz
    bradfitz
    on Jun 30, 2017
    Contributor

    @ALTree I would rather revert CL 36791 now that ReadMemStats is fast (see https://tip.golang.org/doc/go1.9#gc).

    I'll send a CL.

  10. bradfitz commented on Jun 30, 2017

    @bradfitz
    bradfitz
    on Jun 30, 2017
    Contributor

    /cc @josharian @aclements

  11. aclements commented on Jun 30, 2017

    @aclements
    aclements
    on Jun 30, 2017
    Member

    I thought this was fixed a few days ago by https://golang.org/cl/46612?

  12. bradfitz commented on Jun 30, 2017

    @bradfitz
    bradfitz
    on Jun 30, 2017
    Contributor

    Yeah, I was just about to point that out. Looks like that only reverted 1 of the 3 spots.

  13. gopherbot commented on Jun 30, 2017

    @gopherbot
    gopherbot
    on Jun 30, 2017
    Contributor

    CL https://golang.org/cl/47350 mentions this issue.

  14. gopherbot
    added a commit that references this issue on Jun 30, 2017

    testing: revert CL 36791's conditional ReadMemStats

    0ff876a
  15. gopherbot
    closed this as completedin 0ff876aon Jun 30, 2017

  16. ALTree commented on Jun 30, 2017

    @ALTree
    ALTree
    on Jun 30, 2017
    Member

    Ah, thanks for noticing. I didn't notice that that change was partly reverted.

  17. golang
    locked and limited conversation to collaborators on Jun 30, 2018
  18. gopherbot
    added
    FrozenDueToAge
    on Jun 30, 2018
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