Master Go Concurrency: Goroutines, Scheduler, and Synchronization Techniques

1. Using Goroutine to Run Programs

1. Go Concurrency vs Parallelism

Go's concurrency allows a function to run independently of others. When a goroutine is created, the runtime scheduler assigns it to an available logical processor (P) which is bound to an OS thread (M). The scheduler manages goroutine execution time, binds OS threads to logical processors, and maintains runqueues.

Manages all created goroutines and allocates execution time.

Binds OS threads to logical processors.

The scheduler schedules OS threads on physical CPUs, while goroutines are scheduled on logical processors. The three roles are:

M : OS thread (kernel thread).

P : Logical processor, the execution context for goroutines.

G : Goroutine with its own stack and instruction pointer, scheduled by a P.

Each P has a global runqueue. Ready goroutines are placed in this queue and dispatched at scheduling points.

2. Creating Goroutine

Use the go keyword to launch a goroutine. Example:

//example1.go
package main

import (
    "runtime"
    "sync"
    "fmt"
)

var (
    wg sync.WaitGroup
)

func main() {
    // Allocate one logical processor for the scheduler
    runtime.GOMAXPROCS(1)
    wg.Add(2)
    fmt.Printf("Begin Coroutines
")
    go func() {
        defer wg.Done()
        for count := 0; count < 3; count++ {
            for char := 'a'; char < 'a'+26; char++ {
                fmt.Printf("%c ", char)
            }
        }
    }()
    go func() {
        defer wg.Done()
        for count := 0; count < 3; count++ {
            for char := 'A'; char < 'A'+26; char++ {
                fmt.Printf("%c ", char)
            }
        }
    }()
    fmt.Printf("Waiting To Finish
")
    wg.Wait()
}

The program sets runtime.GOMAXPROCS(1), creates two goroutines that print alphabet letters, and waits for them to finish. Output shows the first goroutine completes before the second.

To achieve parallel execution, set two logical processors: runtime.GOMAXPROCS(2) With two processors, the output interleaves the two goroutine outputs.

When only one logical processor is available, goroutines can be forced to yield using runtime.Gosched():

//example2.go
package main

import (
    "runtime"
    "sync"
    "fmt"
)

var (
    wg sync.WaitGroup
)

func main() {
    runtime.GOMAXPROCS(1)
    wg.Add(2)
    fmt.Printf("Begin Coroutines
")
    go func() {
        defer wg.Done()
        for count := 0; count < 3; count++ {
            for char := 'a'; char < 'a'+26; char++ {
                if char == 'k' {
                    runtime.Gosched()
                }
                fmt.Printf("%c ", char)
            }
        }
    }()
    go func() {
        defer wg.Done()
        for count := 0; count < 3; count++ {
            for char := 'A'; char < 'A'+26; char++ {
                if char == 'K' {
                    runtime.Gosched()
                }
                fmt.Printf("%c ", char)
            }
        }
    }()
    fmt.Printf("Waiting To Finish
")
    wg.Wait()
}

This causes the goroutines to alternate execution.

2. Handling Race Conditions

Concurrent programs may encounter race conditions when multiple goroutines access the same resource without synchronization. Example:

//example3.go
package main

import (
    "sync"
    "runtime"
    "fmt"
)

var (
    counter int64
    wg      sync.WaitGroup
)

func addCount() {
    defer wg.Done()
    for count := 0; count < 2; count++ {
        value := counter
        runtime.Gosched()
        value++
        counter = value
    }
}

func main() {
    wg.Add(2)
    go addCount()
    go addCount()
    wg.Wait()
    fmt.Printf("counter: %d
", counter)
}

Running this program may produce counter: 4 or counter: 2 due to unsynchronized reads and writes.

Solutions include:

Using atomic functions.

Using a mutex to protect critical sections.

Using channels.

1. Detecting Race Conditions

Go provides a race detector. Compile with go build -race and run the program.

go build -race example4.go ./example4

The detector reports the lines where the race occurs.

2. Using Atomic Functions

Atomic operations provide safe concurrent access to integers and pointers. Example using atomic.AddInt64:

//example5.go (atomic version)
package main

import (
    "sync"
    "runtime"
    "fmt"
    "sync/atomic"
)

var (
    counter int64
    wg      sync.WaitGroup
)

func addCount() {
    defer wg.Done()
    for count := 0; count < 2; count++ {
        atomic.AddInt64(&counter, 1)
        runtime.Gosched()
    }
}

func main() {
    wg.Add(2)
    go addCount()
    go addCount()
    wg.Wait()
    fmt.Printf("counter: %d
", counter)
}

Other useful atomic functions include atomic.StoreInt64 and atomic.LoadInt64.

3. Using Mutex

A mutex can protect a critical section. Example:

//example5.go (mutex version)
package main

import (
    "sync"
    "runtime"
    "fmt"
)

var (
    counter int
    wg      sync.WaitGroup
    mutex   sync.Mutex
)

func addCount() {
    defer wg.Done()
    for count := 0; count < 2; count++ {
        mutex.Lock()
        value := counter
        runtime.Gosched()
        value++
        counter = value
        mutex.Unlock()
    }
}

func main() {
    wg.Add(2)
    go addCount()
    go addCount()
    wg.Wait()
    fmt.Printf("counter: %d
", counter)
}

Only one goroutine can enter the locked section at a time.

In Go, channels are often the preferred way to avoid race conditions.

3. Sharing Data with Channels

Go follows the CSP model, using channels ( chan) to pass data between goroutines. Channels are created with make:

unbuffered := make(chan int) // unbuffered channel for int
buffered := make(chan string, 10) // buffered channel for string
buffered <- "hello world"
value := <-buffered

Unbuffered channels synchronize send and receive; buffered channels allow storing multiple values.

1. Unbuffered Channels

Unbuffered channels block the sender until a receiver is ready and vice versa. Example simulating a tennis match:

//example6.go
package main

import (
    "sync"
    "fmt"
    "math/rand"
    "time"
)

var wg sync.WaitGroup

func player(name string, court chan int) {
    defer wg.Done()
    for {
        ball, ok := <-court
        if !ok {
            fmt.Printf("Player %s Won
", name)
            return
        }
        n := rand.Intn(100)
        if n%13 == 0 {
            fmt.Printf("Player %s Missed
", name)
            close(court)
            return
        }
        fmt.Printf("Player %s Hit %d
", name, ball)
        ball++
        court <- ball
    }
}

func main() {
    rand.Seed(time.Now().Unix())
    court := make(chan int)
    wg.Add(2)
    go player("candy", court)
    go player("luffic", court)
    court <- 1
    wg.Wait()
}

2. Buffered Channels

Buffered channels hold values until they are received. Sending to a closed channel panics, while receiving from a closed channel yields remaining values. The sender should close the channel.

Summary

Goroutine execution is managed by logical processors, each with its own OS thread and runqueue.

Multiple goroutines can run concurrently on a single logical processor; parallelism requires multiple physical cores.

Goroutines are created with the go keyword.

Race conditions occur when goroutines access shared resources without synchronization.

Mutexes or atomic functions can prevent race conditions.

Channels provide a better solution for safe data sharing.

Unbuffered channels are synchronous; buffered channels are not.