runtime: maps do not shrink after elements removal (delete)

[genez]

What version of Go are you using (go version)?

go version go1.8 windows/amd64

What operating system and processor architecture are you using (go env)?

set GOARCH=amd64
set GOBIN=
set GOEXE=.exe
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOOS=windows
set GOPATH=C:\dev\Go
set GORACE=
set GOROOT=C:\Go
set GOTOOLDIR=C:\Go\pkg\tool\windows_amd64
set GCCGO=gccgo
set CC=gcc
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0
set CXX=g++
set CGO_ENABLED=1
set PKG_CONFIG=pkg-config
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2

What did you do?

See example on playground: https://play.golang.org/p/odsk9F1UH1
(edit: forgot to remove sleeps and changed the number of elements)

What did you expect to see?

removing elements from m1 map should release memory.

What did you see instead?

total allocated memory is always increasing

In the example the issue is not so relevant, but in my production scenario (several maps with more than 1million elements each) I can easily get OOM error, and the process is being killed.
Also I don't know if memstats.Alloc is the right counter to expose here, but I can observe the issue with regular process management tools in linux (e.g. top or htop)

[bradfitz]

/cc @randall77, @josharian

[bradfitz]

/cc @randall77, @josharian

[josharian]

I'm surprised there isn't a dup of this already in the issue tracker.

Yes, maps that shrink permanently currently never get cleaned up after. As usual, the implementation challenge is with iterators.

Maps that shrink and grow repeatedly used to also cause leaks. That was #16070, fixed by CL 25049. I remember hoping when I started on that CL that the same mechanism would be useful for shrinking maps as well, but deciding it wouldn't. Sadly, I no longer remember why. If anyone wants to investigate this issue, I'd start by looking at that CL and thinking about whether that approach could be extended to shrinking maps.

The only available workaround is to make a new map and copy in elements from the old.

[josharian]

I'm surprised there isn't a dup of this already in the issue tracker.

Yes, maps that shrink permanently currently never get cleaned up after. As usual, the implementation challenge is with iterators.

Maps that shrink and grow repeatedly used to also cause leaks. That was #16070, fixed by CL 25049. I remember hoping when I started on that CL that the same mechanism would be useful for shrinking maps as well, but deciding it wouldn't. Sadly, I no longer remember why. If anyone wants to investigate this issue, I'd start by looking at that CL and thinking about whether that approach could be extended to shrinking maps.

The only available workaround is to make a new map and copy in elements from the old.