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runtime: shrink map as elements are deleted #20135

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@genez

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@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)

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    NeedsFixThe path to resolution is known, but the work has not been done.Performancecompiler/runtimeIssues related to the Go compiler and/or runtime.

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