mirror of
https://github.com/prometheus/prometheus.git
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69 lines
2.6 KiB
Go
69 lines
2.6 KiB
Go
// Copyright 2016 The Prometheus Authors
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package local
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import (
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"sync"
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"github.com/prometheus/common/model"
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)
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// fingerprintLocker allows locking individual fingerprints. To limit the number
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// of mutexes needed for that, only a fixed number of mutexes are
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// allocated. Fingerprints to be locked are assigned to those pre-allocated
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// mutexes by their value. Collisions are not detected. If two fingerprints get
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// assigned to the same mutex, only one of them can be locked at the same
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// time. As long as the number of pre-allocated mutexes is much larger than the
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// number of goroutines requiring a fingerprint lock concurrently, the loss in
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// efficiency is small. However, a goroutine must never lock more than one
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// fingerprint at the same time. (In that case a collision would try to acquire
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// the same mutex twice).
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type fingerprintLocker struct {
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fpMtxs []sync.Mutex
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numFpMtxs uint
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}
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// newFingerprintLocker returns a new fingerprintLocker ready for use. At least
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// 1024 preallocated mutexes are used, even if preallocatedMutexes is lower.
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func newFingerprintLocker(preallocatedMutexes int) *fingerprintLocker {
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if preallocatedMutexes < 1024 {
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preallocatedMutexes = 1024
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}
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return &fingerprintLocker{
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make([]sync.Mutex, preallocatedMutexes),
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uint(preallocatedMutexes),
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}
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}
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// Lock locks the given fingerprint.
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func (l *fingerprintLocker) Lock(fp model.Fingerprint) {
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l.fpMtxs[hashFP(fp)%l.numFpMtxs].Lock()
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}
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// Unlock unlocks the given fingerprint.
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func (l *fingerprintLocker) Unlock(fp model.Fingerprint) {
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l.fpMtxs[hashFP(fp)%l.numFpMtxs].Unlock()
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}
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// hashFP simply moves entropy from the most significant 48 bits of the
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// fingerprint into the least significant 16 bits (by XORing) so that a simple
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// MOD on the result can be used to pick a mutex while still making use of
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// changes in more significant bits of the fingerprint. (The fast fingerprinting
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// function we use is prone to only change a few bits for similar metrics. We
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// really want to make use of every change in the fingerprint to vary mutex
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// selection.)
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func hashFP(fp model.Fingerprint) uint {
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return uint(fp ^ (fp >> 32) ^ (fp >> 16))
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}
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