array.go

// ---------
// CPU CACHE
// ---------

// Cores DO NOT access main memory directly but their local caches.
// What store in caches are data and instruction.

// Cache speed from fastest to slowest: L1 -> L2 -> L3 -> main memory.
// Scott Meyers: "If performance matter then the total memeory you have is the total amount of
// caches" -> access to main memory is incredibly slow; practically speaking it might not even be there.

// How do we write code that can be sympathetic with the caching system to make sure that
// we don't have a cache miss or at least, we minimalize cache misses to our fullest potential?

// Processor has a Prefetcher. It predicts what data is needed ahead of time.
// There are different granularity depending on where we are on the machine.
// Our programming model uses a byte. We can read and write to a byte at a time. However, from the
// caching system POV, our granularity is not 1 byte. It is 64 bytes, called a cache line. All
// memory us junked up in this 64 bytes cache line.

// Since the caching mechanism is complex, Prefetcher tries to hide all the latency from us.
// It has to be able to pick up on predictable access pattern to data.
// -> We need to write code that create predictable access pattern to data

// One easy way is to create a contiguous allocation of memory and to iterate over them.
// The array data structure gives us ability to do so.
// From the hardware perspective, array is the most important data structure.
// From Go perspective, slice is. Array is the backing data structure for slice (like Vector in C++).
// Once we allocate an array, whatever it size, every element is equal distant from other element.
// As we iterate over that array, we begin to walk cache line by cache line. As the Prefetcher see
// that access pattern, it can pick it up and hide all the latency from us.

// For example, we have a big nxn matrix. We do LinkedList Traverse, Column Traverse, and Row Traverse
// and benchmark against them.
// Unsurprisingly, Row Traverse has the best performance. It walk through the matrix cache line
// by cache line and create a predictable access pattern.
// Column Traverse does not walk through the matrix cache line by cache line. It looks like random
// access memory pattern. That is why is the slowest among those.
// However, that doesn't explain why the LinkedList Traverse's performance is in the middle. We
// just think that it might perform as poorly as the Column Traverse.
// -> This leads us to another cache: TLB - Translation lookaside buffer. Its job is to maintain
// operating system page and offset to where physical memory is.

// ----------------------------
// Translation lookaside buffer
// ----------------------------

// Back to the different granularity, the caching system moves data in and out the hardware at 64
// bytes at a time. However, the operating system manages memory by paging its 4K (traditional page
// size for an operating system).
// TLB: For every page that we are managing, let's take our virtual memory addresses because that
// that we use (softwares run virtual addresses, its sandbox, that is how we use/share physical
// memory) and map it to the right page and offset for that physical memory.

// A miss on the TLB can be worse than just the cache miss alone.
// The LinkedList is somewhere in between is because the chance of multiple nodes being on the same
// page is probably pretty good. Even though we can get cache misses because cache lines aren't
// necessary in the distance that is predictable, we probably not have so many TLB cache misses.
// In the Column Traverse, not only we have cache misses, we probably have a TLB cache miss on
// every access as well.

// Data-oriented design matters.
// It is not enough to write the most efficient algorithm, how we access our data can have much
// more lasting effect on the performance than the algorithm itself.

package main

import "fmt"

func main() {
	// -----------------------
	// Declare and initialize
	// -----------------------

	// Declare an array of five strings that is initialized to its zero value.
	// Recap: a string is a 2 word data structure: a pointer and a length
	// Since this array is set to its zero value, every string in this array is also set to its
	// zero value, which means that each string has the first word pointed to nil and
	// second word is 0.
	//  -----------------------------
	// | nil | nil | nil | nil | nil |
	//  -----------------------------
	// |  0  |  0  |  0  |  0  |  0  |
	//  -----------------------------
	var strings [5]string

	// At index 0, a string now has a pointer to a backing array of bytes (characters in string)
	// and its length is 5.

	// -----------------
	// What is the cost?
	// -----------------

	// The cost of this assignment is the cost of copying 2 bytes.
	// We have two string values that have pointers to the same backing array of bytes.
	// Therefore, the cost of this assignment is just 2 words.

	//  -----         -------------------
	// |  *  |  ---> | A | p | p | l | e | (1)
	//  -----         -------------------
	// |  5  |                  A
	//  -----                   |
	//                          |
	//                          |
	//     ---------------------
	//    |
	//  -----------------------------
	// |  *  | nil | nil | nil | nil |
	//  -----------------------------
	// |  5  |  0  |  0  |  0  |  0  |
	//  -----------------------------
	strings[0] = "Apple"
	strings[1] = "Orange"
	strings[2] = "Banana"
	strings[3] = "Grape"
	strings[4] = "Plum"

	// ---------------------------------
	// Iterate over the array of strings
	// ---------------------------------

	// Using range, not only we can get the index but also a copy of the value in the array.
	// fruit is now a string value; its scope is within the for statement.
	// In the first iteration, we have the word "Apple". It is a string that has the first word
	// also points to (1) and the second word is 5.
	// So we now have 3 different string value all sharing the same backing array.

	// What are we passing to the Println function?
	// We are using value semantic here. We are not sharing our string value. Println is getting
	// its own copy, its own string value. It means when we get to the Println call, there are now
	// 4 string values all sharing the same backing array.

	// We don't want to take an address of a string.
	// We know the size of a string ahead of time.
	// -> it has the ability to be on the stack
	// -> not creating allocation
	// -> not causing pressure on the GC
	// -> the string has been designed to leverage value mechanic, to stay on the stack, out of the
	// way of creating garbage.
	// -> the only thing that has to be on the heap, if anything is the backing array, which is the
	// one thing that being shared
	fmt.Printf("\n=> Iterate over array\n")
	for i, fruit := range strings {
		fmt.Println(i, fruit)
	}

	// Declare an array of 4 integers that is initialized with some values using literal syntax.
	numbers := [4]int{10, 20, 30, 40}

	// Iterate over the array of numbers using traditional style.
	fmt.Printf("\n=> Iterate over array using traditional style\n")
	for i := 0; i < len(numbers); i++ {
		fmt.Println(i, numbers[i])
	}

	// ---------------------
	// Different type arrays
	// ---------------------

	// Declare an array of 5 integers that is initialized to its zero value.
	var five [5]int

	// Declare an array of 4 integers that is initialized with some values.
	four := [4]int{10, 20, 30, 40}

	fmt.Printf("\n=> Different type arrays\n")
	fmt.Println(five)
	fmt.Println(four)

	// When we try to assign four to five like so five = four, the compiler says that
	// "cannot use four (type [4]int) as type [5]int in assignment"
	// This cannot happen because they have different types (size and representation).
	// The size of an array makes up its type name: [4]int vs [5]int. Just like what we've seen
	// with pointer. The * in *int is not an operator but part of the type name.

	// Unsurprisingly, all array has known size at compiled time.

	// -----------------------------
	// Contiguous memory allocations
	// -----------------------------

	// Declare an array of 5 strings initialized with values.
	six := [6]string{"Annie", "Betty", "Charley", "Doug", "Edward", "Hoanh"}

	// Iterate over the array displaying the value and address of each element.
	// By looking at the output of this Printf function, we can see that this array is truly a
	// contiguous block of memory. We know a string is 2 word and depending on computer
	// architecture, it will have x byte. The distance between two consecutive IndexAddr is exactly
	// x byte.
	// v is its own variable on the stack and it has the same address every single time.
	fmt.Printf("\n=> Contiguous memory allocations\n")
	for i, v := range six {
		fmt.Printf("Value[%s]\tAddress[%p] IndexAddr[%p]\n", v, &v, &six[i])
	}
}