Please answer these questions before submitting your issue. Thanks!

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

go version devel +c5f0104 Wed Apr 25 21:34:15 2018 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/jerrin/.cache/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/jerrin/gocode"
GORACE=""
GOROOT="/work/go"
GOTMPDIR=""
GOTOOLDIR="/work/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build042307601=/tmp/go-build -gno-record-gcc-switches"

What did you do?

When grow() in mheap.go is called, it sets the span entry for every page in the newly created span. This seems unnecessary as the span is anyway going to be put into the free list/treap, and only the first and last span entry need be set for a free span. The following diff is a suggestion to improve this piece of code. The newly added code is similar to how allocSpanLocked() in mheap.go trims the extra pages and puts it back to the free list/treap.

diff --git a/src/runtime/mheap.go b/src/runtime/mheap.go
index b11853c..3b050cf 100644
--- a/src/runtime/mheap.go
+++ b/src/runtime/mheap.go
@@ -923,14 +923,12 @@ func (h *mheap) grow(npage uintptr) bool {
                return false
        }
 
-       // Create a fake "in use" span and free it, so that the
-       // right coalescing happens.
+       // Create a fake span and free it, so that the right coalescing happens.
        s := (*mspan)(h.spanalloc.alloc())
        s.init(uintptr(v), size/pageSize)
-       h.setSpans(s.base(), s.npages, s)
-       atomic.Store(&s.sweepgen, h.sweepgen)
-       s.state = _MSpanInUse
-       h.pagesInUse += uint64(s.npages)
+       h.setSpan(s.base(), s)
+       h.setSpan(s.base()+s.npages*pageSize-1, s)
+       s.state = _MSpanManual
        h.freeSpanLocked(s, false, true, 0)
        return true
 }