_Anthony Starks has remixed my original Google Present based slides using his fantastic Deck presentation tool. You can check out his remix over on his blog,[mindchunk.blogspot.com.au/2014/06/remixing-with-deck][5]._
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I was recently invited to give a talk at Gocon, a fantastic Go conference held semi-annually in Tokyo, Japan.[Gocon 2014][6]was an entirely community-run one day eventcombining training and an afternoon of presentations surrounding the theme of<qstyle="border: 0px; vertical-align: baseline; quotes: none;">Go in production</q>.
The following is the text of my presentation. The original text was structured to force me to speak slowly and clearly, so I have taken the liberty of editing it slightly to be more readable.
I want to thank[Bill Kennedy][7], Minux Ma, and especially[Josh Bleecher Snyder][8], for their assistance in preparing this talk.
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Good afternoon.
My name is David.
I am delighted to be here at Gocon today. I have wanted to come to this conference for two years and I am very grateful to the organisers for extending me the opportunity to present to you today.
When people talk about their decision to learn Go, or use it in their product, they have a variety of answers, but there always three that are at the top of their list
Go’s concurrency primitives are attractive to programmers who come from single threaded scripting languages like Nodejs, Ruby, or Python, or from languages like C++ or Java with their heavyweight threading model.
Ease of deployment.
We have heard today from experienced Gophers who appreciate the simplicity of deploying Go applications.
This is a`Location`type which holds the location of some object in three dimensional space. It is written in Go, so each`Location`consumes exactly 24 bytes of storage.
We can use this type to construct an array type of 1,000`Location`s, which consumes exactly 24,000 bytes of memory.
Inside the array, the`Location`structures are stored sequentially, rather than as pointers to 1,000 Location structures stored randomly.
This is important because now all 1,000`Location`structures are in the cache in sequence, packed tightly together.
The Go compiler inlines a function by treating the body of the function as if it were part of the caller.
Inlining has a cost; it increases binary size.
It only makes sense to inline when the overhead of calling a function is large relative to the work the function does, so only simple functions are candidates for inlining.
Complicated functions are usually not dominated by the overhead of calling them and are therefore not inlined.
After inlining there is no longer a call to`util.Max`, but the behaviour of`Double`is unchanged.
Inlining isn’t exclusive to Go. Almost every compiled or JITed language performs this optimisation. But how does inlining in Go work?
The Go implementation is very simple. When a package is compiled, any small function that is suitable for inlining is marked and then compiled as usual.
Then both the source of the function and the compiled version are stored.
The compiler now knows that the expensive code is unreachable.
Not only does this save the cost of calling Test, it saves compiling or running any of the expensive code that is now unreachable.
The Go compiler can automatically inline functions across files and even across packages. This includes code that calls inlinable functions from the standard library.
There is however another place to allocate memory, and that is the stack.
Unlike C, which forces you to choose if a value will be stored on the heap, via`malloc`, or on the stack, by declaring it inside the scope of the function, Go implements an optimisation called_escape analysis_ .
`Sum`adds the numbers between 1 and 100 and returns the result. This is a rather unusual way to do this, but it illustrates how Escape Analysis works.
Because the numbers slice is only referenced inside`Sum`, the compiler will arrange to store the 100 integers for that slice on the stack, rather than the heap.
There is no need to garbage collect`numbers`, it is automatically freed when`Sum`returns.
Go’s optimisations are always enabled by default. You can see the compiler’s escape analysis and inlining decisions with the`-gcflags=-m`switch.
Because escape analysis is performed at compile time, not run time, stack allocation will always be faster than heap allocation, no matter how efficient your garbage collector is.
I will talk more about the stack in the remaining sections of this talk.
Go has goroutines. These are the foundations for concurrency in Go.
I want to step back for a moment and explore the history that leads us to goroutines.
In the beginning computers ran one process at a time. Then in the 60’s the idea of multiprocessing, or time sharing became popular.
In a time-sharing system the operating systems must constantly switch the attention of the CPU between these processes by recording the state of the current process, then restoring the state of another.
First is the kernel needs to store the contents of all the CPU registers for that process, then restore the values for another process.
The kernel also needs to flush the CPU’s mappings from virtual memory to physical memory as these are only valid for the current process.
Finally there is the cost of the operating system context switch, and the overhead of the scheduler function to choose the next process to occupy the CPU.
There are a surprising number of registers in a modern processor. I have difficulty fitting them on one slide, which should give you a clue how much time it takes to save and restore them.
Because a process switch can occur at any point in a process’ execution, the operating system needs to store the contents of all of these registers because it does not know which are currently in use.
This an example to illustrate some of the scheduling points described in the previous slide.
The thread, depicted by the arrow, starts on the left in the`ReadFile`function. It encounters`os.Open`, which blocks the thread while waiting for the file operation to complete, so the scheduler switches the thread to the goroutine on the right hand side.
Execution continues until the read from the`c`chan blocks, and by this time the`os.Open`call has completed so the scheduler switches the thread back the left hand side and continues to the`file.Read`function, which again blocks on file IO.
The scheduler switches the thread back to the right hand side for another channel operation, which has unblocked during the time the left hand side was running, but it blocks again on the channel send.
Finally the thread switches back to the left hand side as the`Read`operation has completed and data is available.
This slide shows the low level`runtime.Syscall`function which is the base for all functions in the os package.
Any time your code results in a call to the operating system, it will go through this function.
The call to`entersyscall`informs the runtime that this thread is about to block.
This allows the runtime to spin up a new thread which will service other goroutines while this current thread blocked.
This results in relatively few operating system threads per Go process, with the Go runtime taking care of assigning a runnable Goroutine to a free operating system thread.
In the previous section I discussed how goroutines reduce the overhead of managing many, sometimes hundreds of thousands of concurrent threads of execution.
There is another side to the goroutine story, and that is stack management, which leads me to my final topic.
Because the heap and stack overwriting each other would be catastrophic, the operating system usually arranges to place an area of unwritable memory between the stack and the heap to ensure that if they did collide, the program will abort.
This is called a guard page, and effectively limits the stack size of a process, usually in the order of several megabytes.
We’ve discussed that threads share the same address space, so for each thread, it must have its own stack.
Because it is hard to predict the stack requirements of a particular thread, a large amount of memory is reserved for each thread’s stack along with a guard page.
The hope is that this is more than will ever be needed and the guard page will never be hit.
The downside is that as the number of threads in your program increases, the amount of available address space is reduced.
We’ve seen that the Go runtime schedules a large number of goroutines onto a small number of threads, but what about the stack requirements of those goroutines ?
Instead of using guard pages, the Go compiler inserts a check as part of every function call to check if there is sufficient stack for the function to run. If there is not, the runtime can allocate more stack space.
Because of this check, a goroutines initial stack can be made much smaller, which in turn permits Go programmers to treat goroutines as cheap resources.
This is a slide that shows how stacks are managed in Go 1.2.
When`G`calls to`H`there is not enough space for`H`to run, so the runtime allocates a new stack frame from the heap, then runs`H`on that new stack segment. When`H`returns, the stack area is returned to the heap before returning to`G`.
This method of managing the stack works well in general, but for certain types of code, usually recursive code, it can cause the inner loop of your program to straddle one of these stack boundaries.
For example, in the inner loop of your program, function`G`may call`H`many times in a loop,
Each time this will cause a stack split. This is known as the hot split problem.
Values, Inlining, Escape Analysis, Goroutines, and segmented/copying stacks.
These are the five features that I chose to speak about today, but they are by no means the only things that makes Go a fast programming language, just as there more that three reasons that people cite as their reason to learn Go.
As powerful as these five features are individually, they do not exist in isolation.
For example, the way the runtime multiplexes goroutines onto threads would not be nearly as efficient without growable stacks.
Inlining reduces the cost of the stack size check by combining smaller functions into larger ones.
Escape analysis reduces the pressure on the garbage collector by automatically moving allocations from the heap to the stack.
Escape analysis is also provides better cache locality.
Without growable stacks, escape analysis might place too much pressure on the stack.
