victoriaMetrics中的一些Sao操作

victoriaMetrics中的一些Sao操作

快速获取当前时间

victoriaMetrics中有一个fasttime库，用于快速获取当前的Unix时间，实现其实挺简单，就是在后台使用一个goroutine不断以1s为周期刷新表示当前时间的变量currentTimestamp，获取的时候直接原子加载该变量即可。其性能约是time.Now()的8倍。

其核心方式就是将主要任务放到后台运行，通过一个中间变量来传递运算结果，以此来通过异步的方式提升性能，但需要业务能包容一定的精度偏差。

func init() {
	go func() {
		ticker := time.NewTicker(time.Second)
		defer ticker.Stop()
		for tm := range ticker.C {
			t := uint64(tm.Unix())
			atomic.StoreUint64(&currentTimestamp, t)
		}
	}()
}

var currentTimestamp = uint64(time.Now().Unix())

// UnixTimestamp returns the current unix timestamp in seconds.
//
// It is faster than time.Now().Unix()
func UnixTimestamp() uint64 {
	return atomic.LoadUint64(&currentTimestamp)
}

计算结构体的哈希值

hashUint64函数中使用xxhash.Sum64计算了结构体Key的哈希值。通过unsafe.Pointer将指针转换为*[]byte类型，byte数组的长度为unsafe.Sizeof(*k)，unsafe.Sizeof()返回结构体的字节大小。

如果一个数据为固定的长度，如h的类型为uint64，则可以直接指定长度为8进行转换，如：bp:=([8]byte)(unsafe.Pointer(&h))

需要注意的是unsafe.Sizeof()返回的是数据结构的大小而不是其指向内容的数据大小，如下返回的slice大小为24，为slice首部数据结构SliceHeader的大小，而不是其引用的数据大小(可以使用len获取slice引用的数据大小)。此外如果结构体中有指针，则转换成的byte中存储的也是指针存储的地址。

slice := []int{1,2,3,4,5,6,7,8,9,10}
fmt.Println(unsafe.Sizeof(slice)) //24

type Key struct {
	Part interface{}
	Offset uint64
}

func (k *Key) hashUint64() uint64 {
	buf := (*[unsafe.Sizeof(*k)]byte)(unsafe.Pointer(k))
	return xxhash.Sum64(buf[:])
}

将字符串添加到已有的[]byte中

使用如下方式即可：

str := "1231445"
arr := []byte{1, 2, 3}
arr = append(arr, str...)

将int64的数组转换为byte数组

直接操作了底层的SliceHeader

func int64ToByteSlice(a []int64) (b []byte) {
   sh := (*reflect.SliceHeader)(unsafe.Pointer(&b))
   sh.Data = uintptr(unsafe.Pointer(&a[0]))
   sh.Len = len(a) * int(unsafe.Sizeof(a[0]))
   sh.Cap = sh.Len
   return
}

并发访问的sync.WaitGroup

并发访问的sync.WaitGroup的目的是为了在运行时添加需要等待的goroutine

// WaitGroup wraps sync.WaitGroup and makes safe to call Add/Wait
// from concurrent goroutines.
//
// An additional limitation is that call to Wait prohibits further calls to Add
// until return.
type WaitGroup struct {
	sync.WaitGroup
	mu sync.Mutex
}

// Add registers n additional workers. Add may be called from concurrent goroutines.
func (wg *WaitGroup) Add(n int) {
	wg.mu.Lock()
	wg.WaitGroup.Add(n)
	wg.mu.Unlock()
}

// Wait waits until all the goroutines call Done.
//
// Wait may be called from concurrent goroutines.
//
// Further calls to Add are blocked until return from Wait.
func (wg *WaitGroup) Wait() {
	wg.mu.Lock()
	wg.WaitGroup.Wait()
	wg.mu.Unlock()
}

// WaitAndBlock waits until all the goroutines call Done and then prevents
// from new goroutines calling Add.
//
// Further calls to Add are always blocked. This is useful for graceful shutdown
// when other goroutines calling Add must be stopped.
//
// wg cannot be used after this call.
func (wg *WaitGroup) WaitAndBlock() {
	wg.mu.Lock()
	wg.WaitGroup.Wait()

	// Do not unlock wg.mu, so other goroutines calling Add are blocked.
}

// There is no need in wrapping WaitGroup.Done, since it is already goroutine-safe.

时间池

高频次创建timer会消耗一定的性能，为了减少某些情况下的性能损耗，可以使用sync.Pool来回收利用创建的timer

// Get returns a timer for the given duration d from the pool.
//
// Return back the timer to the pool with Put.
func Get(d time.Duration) *time.Timer {
	if v := timerPool.Get(); v != nil {
		t := v.(*time.Timer)
		if t.Reset(d) {
			logger.Panicf("BUG: active timer trapped to the pool!")
		}
		return t
	}
	return time.NewTimer(d)
}

// Put returns t to the pool.
//
// t cannot be accessed after returning to the pool.
func Put(t *time.Timer) {
	if !t.Stop() {
		// Drain t.C if it wasn't obtained by the caller yet.
		select {
		case <-t.C:
		default:
		}
	}
	timerPool.Put(t)
}

var timerPool sync.Pool

访问限速

victoriaMetrics的vminsert作为vmagent和vmstorage之间的组件，接收vmagent的流量并将其转发到vmstorage。在vmstorage卡死、处理过慢或下线的情况下，有可能会导致无法转发流量，进而造成vminsert CPU和内存飙升，造成组件故障。为了防止这种情况，vminsert使用了限速器，当接收到的流量激增时，可以在牺牲一部分数据的情况下保证系统的稳定性。

victoriaMetrics的源码中对限速器有如下描述：

Limit the number of conurrent f calls in order to prevent from excess memory usage and CPU thrashing

限速器使用了两个参数：maxConcurrentInserts和maxQueueDuration，前者给出了突发情况下可以处理的最大请求数，后者给出了某个请求的最大超时时间。需要注意的是Do(f func() error)是异步执行的，而ch又是全局的，因此会异步等待其他请求释放资源(struct{})。

可以看到限速器使用了指标来指示当前的限速状态。同时使用cgroup.AvailableCPUs()*4 (即runtime.GOMAXPROCS(-1)*4)来设置默认的maxConcurrentInserts长度。

当该限速器用在处理如http请求时，该限速器并不能限制底层上送的请求，其限制的是对请求的处理。在高流量业务处理中，这也是最消耗内存的地方，通常包含数据读取、内存申请拷贝等。底层的数据受/proc/sys/net/core/somaxconn和socket缓存区的限制。

var (
	maxConcurrentInserts = flag.Int("maxConcurrentInserts", cgroup.AvailableCPUs()*4, "The maximum number of concurrent inserts. Default value should work for most cases, "+
		"since it minimizes the overhead for concurrent inserts. This option is tigthly coupled with -insert.maxQueueDuration")
	maxQueueDuration = flag.Duration("insert.maxQueueDuration", time.Minute, "The maximum duration for waiting in the queue for insert requests due to -maxConcurrentInserts")
)

// ch is the channel for limiting concurrent calls to Do.
var ch chan struct{}

// Init initializes concurrencylimiter.
//
// Init must be called after flag.Parse call.
func Init() {
	ch = make(chan struct{}, *maxConcurrentInserts) //初始化limiter，最大突发并行请求量为maxConcurrentInserts
}

// Do calls f with the limited concurrency.
func Do(f func() error) error {
	// Limit the number of conurrent f calls in order to prevent from excess
	// memory usage and CPU thrashing.
	select {
	case ch <- struct{}{}: //在channel中添加一个元素，表示开始处理一个请求
		err := f() //阻塞等大请求处理结束
		<-ch //请求处理完之后释放channel中的一个元素，释放出的空间可以用于处理下一个请求
		return err
	default:
	}

    //如果当前达到处理上限maxConcurrentInserts，则需要等到其他Do(f func() error)释放资源。
	// All the workers are busy.
	// Sleep for up to *maxQueueDuration.
	concurrencyLimitReached.Inc()
	t := timerpool.Get(*maxQueueDuration) //获取一个timer，设置等待超时时间为 maxQueueDuration
	select {
	case ch <- struct{}{}: //在maxQueueDuration时间内等待其他请求释放资源，如果获取到资源，则回收timer，继续处理
		timerpool.Put(t)
		err := f()
		<-
		return err
	case <-t.C: //在maxQueueDuration时间内没有获取到资源，定时器超时后回收timer，丢弃请求并返回错误信息
		timerpool.Put(t)
		concurrencyLimitTimeout.Inc()
		return &httpserver.ErrorWithStatusCode{
			Err: fmt.Errorf("cannot handle more than %d concurrent inserts during %s; possible solutions: "+
				"increase `-insert.maxQueueDuration`, increase `-maxConcurrentInserts`, increase server capacity", *maxConcurrentInserts, *maxQueueDuration),
			StatusCode: http.StatusServiceUnavailable,
		}
	}
}

var (
	concurrencyLimitReached = metrics.NewCounter(`vm_concurrent_insert_limit_reached_total`)
	concurrencyLimitTimeout = metrics.NewCounter(`vm_concurrent_insert_limit_timeout_total`)

	_ = metrics.NewGauge(`vm_concurrent_insert_capacity`, func() float64 {
		return float64(cap(ch))
	})
	_ = metrics.NewGauge(`vm_concurrent_insert_current`, func() float64 {
		return float64(len(ch))
	})
)

优先级控制

victoriaMetrics的pacelimiter库实现了优先级控制。主要方法由Inc、Dec和WaitIfNeeded。低优先级任务需要调用WaitIfNeeded方法，如果此时有高优先级任务(调用Inc方法)，则低优先级任务需要等待高优先级任务结束(调用Dec方法)之后才能继续执行。

// PaceLimiter throttles WaitIfNeeded callers while the number of Inc calls is bigger than the number of Dec calls.
//
// It is expected that Inc is called before performing high-priority work,
// while Dec is called when the work is done.
// WaitIfNeeded must be called inside the work which must be throttled (i.e. lower-priority work).
// It may be called in the loop before performing a part of low-priority work.
type PaceLimiter struct {
	mu          sync.Mutex
	cond        *sync.Cond
	delaysTotal uint64
	n           int32
}

// New returns pace limiter that throttles WaitIfNeeded callers while the number of Inc calls is bigger than the number of Dec calls.
func New() *PaceLimiter {
	var pl PaceLimiter
	pl.cond = sync.NewCond(&pl.mu)
	return &pl
}

// Inc increments pl.
func (pl *PaceLimiter) Inc() {
	atomic.AddInt32(&pl.n, 1)
}

// Dec decrements pl.
func (pl *PaceLimiter) Dec() {
	if atomic.AddInt32(&pl.n, -1) == 0 {
		// Wake up all the goroutines blocked in WaitIfNeeded,
		// since the number of Dec calls equals the number of Inc calls.
		pl.cond.Broadcast()
	}
}

// WaitIfNeeded blocks while the number of Inc calls is bigger than the number of Dec calls.
func (pl *PaceLimiter) WaitIfNeeded() {
	if atomic.LoadInt32(&pl.n) <= 0 {
		// Fast path - there is no need in lock.
		return
	}
	// Slow path - wait until Dec is called.
	pl.mu.Lock()
	for atomic.LoadInt32(&pl.n) > 0 {
		pl.delaysTotal++
		pl.cond.Wait()
	}
	pl.mu.Unlock()
}

// DelaysTotal returns the number of delays inside WaitIfNeeded.
func (pl *PaceLimiter) DelaysTotal() uint64 {
	pl.mu.Lock()
	n := pl.delaysTotal
	pl.mu.Unlock()
	return n
}