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scheduler.go
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scheduler.go
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// Copyright 2016 Google Inc. All Rights Reserved.
//
// 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 scheduler provides the scheduler, which takes requests of particular types
// (e.g. read, write, open) and returns how long they should wait before completing.
package scheduler
import (
"slowfs/slowfs"
"time"
)
// Scheduler determines how long operations should take given a description of a physical medium.
type Scheduler struct {
dc *deviceContext
readWriteQueue *readWriteQueue
requests chan *requestData
}
// New creates a new Scheduler using the given DeviceConfig to help compute how long requests
// should take.
func New(config *slowfs.DeviceConfig) *Scheduler {
dc := newDeviceContext(config)
scheduler := &Scheduler{
dc: dc,
readWriteQueue: newReadWriteQueue(dc),
requests: make(chan *requestData, 10),
}
go scheduler.serveRequests()
return scheduler
}
type requestData struct {
req *Request
responseChannel chan time.Duration
}
// Schedule schedules a new request and returns how long the request should take.
// N.B. this can block.
func (s *Scheduler) Schedule(req *Request) time.Duration {
ch := make(chan time.Duration, 1)
s.requests <- &requestData{req, ch}
return <-ch
}
// Main event loop to serve requests.
func (s *Scheduler) serveRequests() {
for {
select {
case reqData := <-s.requests:
req, resp := reqData.req, reqData.responseChannel
switch req.Type {
case ReadRequest, WriteRequest:
s.readWriteQueue.push(reqData)
default:
resp <- s.dc.computeTime(req)
s.dc.execute(req)
}
case <-s.readWriteQueue.responseChannel():
reqData := s.readWriteQueue.pop(time.Now())
if reqData != nil {
reqData.responseChannel <- s.dc.computeTime(reqData.req)
s.dc.execute(reqData.req)
}
}
// This needs to be called every loop, since executing a request can change how long a
// read or write request on the front of the queue would take.
s.readWriteQueue.scheduleResponse(time.Now())
}
}