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flow_scheduler.go
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flow_scheduler.go
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// Copyright 2016 The Cockroach 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.
//
// Author: Radu Berinde (radu@cockroachlabs.com)
package distsqlrun
import (
"container/list"
"github.com/cockroachdb/cockroach/pkg/util/log"
"github.com/cockroachdb/cockroach/pkg/util/stop"
"github.com/cockroachdb/cockroach/pkg/util/syncutil"
)
const maxRunningFlows = 500
const flowDoneChanSize = 8
// flowScheduler manages running flows and decides when to queue and when to
// start flows. The main interface it presents is ScheduleFlows, which passes a
// flow to be run.
type flowScheduler struct {
log.AmbientContext
stopper *stop.Stopper
flowDoneCh chan *Flow
mu struct {
syncutil.Mutex
numRunning int
queue *list.List
}
}
func newFlowScheduler(ambient log.AmbientContext, stopper *stop.Stopper) *flowScheduler {
fs := &flowScheduler{
AmbientContext: ambient,
stopper: stopper,
flowDoneCh: make(chan *Flow, flowDoneChanSize),
}
fs.mu.queue = list.New()
return fs
}
func (fs *flowScheduler) canRunFlow(_ *Flow) bool {
// TODO(radu): we will have more complex resource accounting (like memory).
// For now we just limit the number of concurrent flows.
return fs.mu.numRunning < maxRunningFlows
}
// runFlowNow starts the given flow; does not wait for the flow to complete.
func (fs *flowScheduler) runFlowNow(f *Flow) {
fs.mu.numRunning++
f.Start(func() { fs.flowDoneCh <- f })
// TODO(radu): we could replace the WaitGroup with a structure that keeps a
// refcount and automatically runs Cleanup() when the count reaches 0.
go func() {
f.Wait()
f.Cleanup()
}()
}
// ScheduleFlow is the main interface of the flow scheduler: it runs or enqueues
// the given flow.
func (fs *flowScheduler) ScheduleFlow(f *Flow) error {
return fs.stopper.RunTask(func() {
fs.mu.Lock()
defer fs.mu.Unlock()
if fs.canRunFlow(f) {
fs.runFlowNow(f)
} else {
fs.mu.queue.PushBack(f)
}
})
}
// Start launches the main loop of the scheduler.
func (fs *flowScheduler) Start() {
fs.stopper.RunWorker(func() {
stopped := false
fs.mu.Lock()
defer fs.mu.Unlock()
for {
if stopped && fs.mu.numRunning == 0 {
// TODO(radu): somehow error out the flows that are still in the queue.
return
}
fs.mu.Unlock()
select {
case <-fs.flowDoneCh:
fs.mu.Lock()
fs.mu.numRunning--
if !stopped {
if frElem := fs.mu.queue.Front(); frElem != nil {
f := frElem.Value.(*Flow)
fs.mu.queue.Remove(frElem)
fs.runFlowNow(f)
}
}
case <-fs.stopper.ShouldStop():
fs.mu.Lock()
stopped = true
}
}
})
}