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

$ go version
go version go1.16.3 darwin/arm64

Does this issue reproduce with the latest release?

yes

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

$ go env
GO111MODULE=""
GOARCH="arm64"
GOBIN=""
GOCACHE="/Users/andig/Library/Caches/go-build"
GOENV="/Users/andig/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/andig/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/andig/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/opt/homebrew/Cellar/go/1.16.3/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/opt/homebrew/Cellar/go/1.16.3/libexec/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.16.3"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/andig/htdocs/evcc/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/sv/rs_453y57xj86xsbz3kw1mbc0000gn/T/go-build87690164=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Investigating a server application with a larger number of active log.Loggers (with different prefixes) I've noticed that memory consumption kept growing and pprof showed memory consumed in log.Output. It seems that the reason is the allocated, grow-only buffer in conjunction with extensive logging of e.g. HTTP traffic, see

What did you expect to see?

Flat memory consumption for logging, almost flat total.

What did you see instead?

Growing memory consumption as more live objects with expensive loggers are added.

I would like to suggest to make log.Logger use a sync pool like this:

var bufPool = sync.Pool{
	New: func() interface{} {
		return []byte{}
	},
}

where Output becomes:

	// l.buf = l.buf[:0]
	buf := bufPool.Get().([]byte)

        // as before
	l.formatHeader(&buf, now, file, line)
	buf = append(buf, s...)
	if len(s) == 0 || s[len(s)-1] != '\n' {
		buf = append(buf, '\n')
	}
	_, err := l.out.Write(buf)

        // put small buffers back
	if cap(buf) < 256 {
		bufPool.Put(buf[:0])
	}
	return err

That way particulary long log lines would no longer cause memory consumption at the cost of additional allocations. Might also chose a higher limit for putting resources back to the pool.

Would be happy to provide a PR.