2014/go1.3.slide

Toward Go 1.3
(and beyond)

Andrew Gerrand
Gopher
@enneff
http://golang.org


* Go 1.3

Code freeze is March 1, 2014.

Release is June 1, 2014.

(Six months after Go 1.2, released December 1, 2013.)


* A to-do list

After Go 1.2 the Go contributors compiled a to-do list:

.link http://golang.org/s/go13todo

The list is aspirational; not all of it will get done.

This talk is based on that list.


* 100% precise GC

Finally!


* Copying stacks (1/2)

.link http://golang.org/s/contigstacks

Go 1.2's stack split mechanism has a "hot split" problem.

Copying (or "contiguous") stacks are grown by reallocation and copying.
Resolves the "hot split" problem.
Makes smaller initial stacks practical - more goroutines in the same space.

* Copying stacks (2/2)

.image go1.3/json.png


* Dmitry's bag of performance tricks

Runtime changes:

- increase page size to 8K (~10% GC less pause time)
- do not collect GC roots explicitly (~6% GC less pause time)
- prefetch next block in mallocgc (~2% less CPU)
- smarter slice grow (2-20% less CPU)
- combine small NoScan allocations (10% faster json benchmark)
- do not zero terminate strings (1% fewer allocs json benchmark)
- remove locks from netpoll hotpaths (~5% faster TCP)
- allocate goroutine ids in batches (8-66% faster goroutine creation)
- use lock-free ring for work queues (5-40% faster goroutine scheduling)
- per-P defer pool (memory savings for programs with many goroutines)

And many more to come...


* Channel rewrite

.link http://golang.org/s/go13chan

Goals:

- make single-threaded (non-contended) channel operations faster
- make contended buffered (producer/consumer) channel operations faster
- make non-blocking failing operations (e.g. checking of "stop" channel) faster
- make chan semaphores (chan struct{}) faster
- make select statements faster

Non-goals:

- make channels completely lock-free (this would significantly complicate implementation and make it slower for common cases)
- make contended synchronous channel operations faster


* sync.Pool (1/2)

Many Go libraries include custom thread-safe free lists, like this:

	var objPool = make(chan *Object, 10)

	func obj() *Object {
		select {
		case p := <-objPool:
			return p
		default:
		}
		return NewObject()
	}

	func objPut(p *Object) {
		select {
		case objPool <- p:
		default:
		}
	}
  
	p := obj()
	// use p
	objPut(p)

* sync.Pool (2/2)

The `sync.Pool` type provides a general thread-safe global free list.

It allows the runtime to reclaim entries when appropriate
(for example, during garbage collection).

	var objPool = sync.Pool{
		New: func() interface{} {
			return NewObject()
		},
	}
  
	p := objPool.Get().(*Object)
	// use p
	objPool.Put(p)

This is an experimental type and might not be released.


* Native Client port

.link http://golang.org/s/go13nacl

Native Client (NaCl) is a restricted execution environment for x86 binaries.

Notably used to run compiled binaries inside Google Chrome.
NaCl also provides a tool for executing command-line binaries

Go 1.3 targets that command-line tool for 32-bit and 64-bit x86 architectures.
(NaCl supports 32-bit ARM, but we have no plans to support it.)

The [[http://play.golang.org][Go Playground]] uses the NaCl tool chain to safely execute untrusted programs.

The NaCl tool chain includes the fake time, network, and file system capabilities of the playground.


* OS ports

Solaris: work in progress, on track for Go 1.3.

DragonflyBSD: work is done, looking for a maintainer.

Plan 9: still not finished.

darwin/arm, android/arm: a contributor is working on these, some way to go.


* The go command and fsnotify

.link http://golang.org/s/go13fsnotify

In Go 1.2, `go` `build` stats every dependent source file to see whether they have changed.

This is a big chunk of total build time.

The proposed "go background" command starts a daemon that watches source files for changes.

When building, the `go` commands can ask the daemon which files have changed.

A new `os/fsnotify` package will be added to the standard library to support the `go` command.

A proposed interface is discussed here:

.link http://golang.org/cl/48310043


* Support for linking against Objective C code

The Go 1.2 tool chain can link against C++ code using `cgo` (but you need to write a small C bridge into the C++ code).

The same can be done for Objective C code, with some modifications to the go tool.

This will make it easier to write native OS X applications.


* Address binary bloat

.link http://golang.org/issue/6853

Go binaries are getting pretty big. Rob ran an experiment:

	As an experiment, I built "hello, world" at the release points for go 1.0. 1.1, and 1.2.
	Here are the binary's sizes:

	% ls -l x.1.?
	-rwxr-xr-x  1 r  staff  1191952 Nov 30 10:25 x.1.0
	-rwxr-xr-x  1 r  staff  1525936 Nov 30 10:20 x.1.1
	-rwxr-xr-x  1 r  staff  2188576 Nov 30 10:18 x.1.2

Go binaries contain several sets of debugging symbols (for gdb, profilers, reflection, etc).

We intend to rationalize these as part of some work on the linker.
Speaking of which...


* Linker overhaul (1/3)

.link http://golang.org/s/go13linker

The `gc` tool chain is a bit unconventional.

The compilers don't emit machine code but an intermediate assembly language.

The linker translates it into machine code.

The packages can be compiled in parallel by independent runs of the compiler,
but the linking must be done by a single linker process after compilation is complete.

The `gc` linker has become a bottleneck in building programs
because it does more work than a typical linker.

* Linker overhaul (2/3)

The Go 1.2 linker's job can be split into two parts:

- translate an input stream of pseudo-instructions into executable code, data blocks, and a list of relocations,
- delete dead code, merge what's left, resolve relocations, and generate a few whole-program data structures.

.image go1.3/liblink1.png

* Linker overhaul (3/3)

In Go 1.3, much of the old linker is moved to a `liblink` library that is then used by assemblers and compilers (`6a`, `6c`, `6g`, etc). This allows more work to be done in parallel.

.image go1.3/liblink2.png

And because the linker is much simpler now, we can rewrite it in Go.


* Compiler overhaul

.link http://golang.org/s/go13compiler

The "gc" tool chain is based on the Plan 9 C compilers.

The assemblers, C compilers, and linkers were lifted wholesale.

The Go compilers are new C programs that fit into that tool chain.

Wouldn't it be nice to have a Go compiler written in Go?

* Compiler overhaul: why C then?

Many benefits to writing the compiler in C:

- Go did not exist
- Once Go did exist, it changed often

Today, Go does exist and is stable as of Go 1.
These benefits not as relevant now.

* Compiler overhaul: why Go now?

The benefits of a Go-based compiler:

- Go code is easier to write and debug
- Go has better support for modularity, automated rewriting, unit testing, and profiling
- Go programmers are more likely to work on a compiler written in Go
- Go code is easier to parallelize
- Go is more fun!

* Compiler overhaul: the plan

Not a rewrite.

Translate the C compilers to Go.

Write and use an automatic translator to do this.

Start the process with Go 1.3 and continue in future releases.

* Compiler overhaul: five phases

- Develop and debug the translator.
- Translate the C to Go and delete the C code.
- Clean up and document the code, add unit tests. (Target Go 1.4)
- Profile and optimize the compiler and split it into packages.
- Replace the front end with `go/parser` and `go/types`. (Maybe with new versions of those packages.)

* Compiler overhaul: bootstrapping

Must have a way to build the compiler from scratch.

Our plan is that the Go 1.3 compiler must compile using Go 1.2, and Go 1.4 must compile with Go 1.3, and so on.

Write a shell script to do this automatically. Bootstrap once per machine.

This scales poorly over time, so we might write a back end for the compiler that generates C code, and keep the C version of the compiler sources checked in.

* Compiler overhaul: alternatives

Write new compilers from scratch?

- The existing compilers are well-tested and handle many subtle cases well; would be foolish to throw away 10 man-years of effort.

Translate the compiler manually?

- Translation is tedious and error-prone, mistakes are subtle and hard to find. Can continue to work on existing compilers while writing the translator.

Translate just the back ends and connect to `go/parser` and `go/types` immediately?

- The existing APIs are very different; too much work to undertake at once.

Discard the current compilers and use gccgo (or `go/parser` and `go/types` and LLVM)?

- The current compilers are a large part of our flexibility. Tying Go to large C/C++ projects like GCC or LLVM hurts that flexibility.


* Lots of small things

As with previous releases, we'll see a long tail of small fixes and changes.