net: GoLookupIPNoSuchHost performance regression

[quasilyte]

Commit 672729e (https://golang.org/cl/37879) introduced performance regression for "no such host" case.

goos: linux
goarch: amd64
pkg: net
BenchmarkGoLookupIPNoSuchHost-4               30          43958216 ns/op
BenchmarkGoLookupIPNoSuchHost-4               30          43639313 ns/op
BenchmarkGoLookupIPNoSuchHost-4               30          58653180 ns/op
BenchmarkGoLookupIPNoSuchHost-4               30          49881501 ns/op
BenchmarkGoLookupIPNoSuchHost-4               30          52573745 ns/op

Before that commit:

goos: linux
goarch: amd64
pkg: net
BenchmarkGoLookupIPNoSuchHost-4             2000            681589 ns/op
BenchmarkGoLookupIPNoSuchHost-4             2000            717598 ns/op
BenchmarkGoLookupIPNoSuchHost-4             2000            688904 ns/op
BenchmarkGoLookupIPNoSuchHost-4             2000            690374 ns/op
BenchmarkGoLookupIPNoSuchHost-4             2000            780634 ns/op

In other words:

name                    old time/op  new time/op   delta
GoLookupIPNoSuchHost-4   712µs ±10%  53484µs ±11%  +7413.71%  (p=0.008 n=5+5)

Needs investigation.

[odeke-em]

Thank you for reporting this @quasilyte!

/cc @iangudger @bradfitz

[iangudger]

That is a big jump. I will look into it. I will admit that I mostly optimized for the non-error case.

Edit: How do I run this benchmark?

[quasilyte]

@iangudger, the simplest way is go test -bench=GoLookupIPNoSuchHost net. Is this what you're asking? See also: benchstat.

[iangudger]

I have been able to reproduce the issue. I will start debugging.

[iangudger]

I found the issue. I will try to send out a fix and some benchmarks tomorrow.

[iangudger]

Go 10.10.2:

BenchmarkGoLookupIP-12                        	   10000	    174883 ns/op   11450 B/op	     163 allocs/op
BenchmarkGoLookupIPNoSuchHost-12              	    3000	    670140 ns/op   52189 B/op	     544 allocs/op
BenchmarkGoLookupIPWithBrokenNameServer-12    	       1	5002568137 ns/op  163792 B/op	     375 allocs/op

tip:

BenchmarkGoLookupIP-12                        	   10000	    165501 ns/op    8585 B/op	      94 allocs/op
BenchmarkGoLookupIPNoSuchHost-12              	    1000	   1204117 ns/op   83661 B/op	     674 allocs/op
BenchmarkGoLookupIPWithBrokenNameServer-12    	       1	5002629186 ns/op  159128 B/op	     275 allocs/op

after patch:

BenchmarkGoLookupIP-12                        	   10000	    158102 ns/op    8585 B/op	      94 allocs/op
BenchmarkGoLookupIPNoSuchHost-12              	    2000	    645364 ns/op   42990 B/op	     356 allocs/op
BenchmarkGoLookupIPWithBrokenNameServer-12    	       1	5002163437 ns/op  159144 B/op	     275 allocs/op

[gopherbot]

Change https://golang.org/cl/113815 mentions this issue: net: fix DNS NXDOMAIN performance regression

[gopherbot]

Change https://golang.org/cl/133675 mentions this issue: net: fail fast for DNS rcode success with no answers of requested type