locker.go

// Copyright 2016 The Prometheus Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package local

import (
	"sync"
	"unsafe"

	"github.com/prometheus/common/model"
)

const (
	cacheLineSize = 64
)

// Avoid false sharing when using array of mutexes.
type paddedMutex struct {
	sync.Mutex
	pad [cacheLineSize - unsafe.Sizeof(sync.Mutex{})]byte
}

// fingerprintLocker allows locking individual fingerprints. To limit the number
// of mutexes needed for that, only a fixed number of mutexes are
// allocated. Fingerprints to be locked are assigned to those pre-allocated
// mutexes by their value. Collisions are not detected. If two fingerprints get
// assigned to the same mutex, only one of them can be locked at the same
// time. As long as the number of pre-allocated mutexes is much larger than the
// number of goroutines requiring a fingerprint lock concurrently, the loss in
// efficiency is small. However, a goroutine must never lock more than one
// fingerprint at the same time. (In that case a collision would try to acquire
// the same mutex twice).
type fingerprintLocker struct {
	fpMtxs    []paddedMutex
	numFpMtxs uint
}

// newFingerprintLocker returns a new fingerprintLocker ready for use.  At least
// 1024 preallocated mutexes are used, even if preallocatedMutexes is lower.
func newFingerprintLocker(preallocatedMutexes int) *fingerprintLocker {
	if preallocatedMutexes < 1024 {
		preallocatedMutexes = 1024
	}
	return &fingerprintLocker{
		make([]paddedMutex, preallocatedMutexes),
		uint(preallocatedMutexes),
	}
}

// Lock locks the given fingerprint.
func (l *fingerprintLocker) Lock(fp model.Fingerprint) {
	l.fpMtxs[hashFP(fp)%l.numFpMtxs].Lock()
}

// Unlock unlocks the given fingerprint.
func (l *fingerprintLocker) Unlock(fp model.Fingerprint) {
	l.fpMtxs[hashFP(fp)%l.numFpMtxs].Unlock()
}

// hashFP simply moves entropy from the most significant 48 bits of the
// fingerprint into the least significant 16 bits (by XORing) so that a simple
// MOD on the result can be used to pick a mutex while still making use of
// changes in more significant bits of the fingerprint. (The fast fingerprinting
// function we use is prone to only change a few bits for similar metrics. We
// really want to make use of every change in the fingerprint to vary mutex
// selection.)
func hashFP(fp model.Fingerprint) uint {
	return uint(fp ^ (fp >> 32) ^ (fp >> 16))
}