flowcontrol.go

Documentation: google.golang.org/grpc/internal/transport

     1  /*
     2   *
     3   * Copyright 2014 gRPC authors.
     4   *
     5   * Licensed under the Apache License, Version 2.0 (the "License");
     6   * you may not use this file except in compliance with the License.
     7   * You may obtain a copy of the License at
     8   *
     9   *     http://www.apache.org/licenses/LICENSE-2.0
    10   *
    11   * Unless required by applicable law or agreed to in writing, software
    12   * distributed under the License is distributed on an "AS IS" BASIS,
    13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    14   * See the License for the specific language governing permissions and
    15   * limitations under the License.
    16   *
    17   */
    18  
    19  package transport
    20  
    21  import (
    22  	"fmt"
    23  	"math"
    24  	"sync"
    25  	"sync/atomic"
    26  )
    27  
    28  // writeQuota is a soft limit on the amount of data a stream can
    29  // schedule before some of it is written out.
    30  type writeQuota struct {
    31  	quota int32
    32  	// get waits on read from when quota goes less than or equal to zero.
    33  	// replenish writes on it when quota goes positive again.
    34  	ch chan struct{}
    35  	// done is triggered in error case.
    36  	done <-chan struct{}
    37  	// replenish is called by loopyWriter to give quota back to.
    38  	// It is implemented as a field so that it can be updated
    39  	// by tests.
    40  	replenish func(n int)
    41  }
    42  
    43  func newWriteQuota(sz int32, done <-chan struct{}) *writeQuota {
    44  	w := &writeQuota{
    45  		quota: sz,
    46  		ch:    make(chan struct{}, 1),
    47  		done:  done,
    48  	}
    49  	w.replenish = w.realReplenish
    50  	return w
    51  }
    52  
    53  func (w *writeQuota) get(sz int32) error {
    54  	for {
    55  		if atomic.LoadInt32(&w.quota) > 0 {
    56  			atomic.AddInt32(&w.quota, -sz)
    57  			return nil
    58  		}
    59  		select {
    60  		case <-w.ch:
    61  			continue
    62  		case <-w.done:
    63  			return errStreamDone
    64  		}
    65  	}
    66  }
    67  
    68  func (w *writeQuota) realReplenish(n int) {
    69  	sz := int32(n)
    70  	a := atomic.AddInt32(&w.quota, sz)
    71  	b := a - sz
    72  	if b <= 0 && a > 0 {
    73  		select {
    74  		case w.ch <- struct{}{}:
    75  		default:
    76  		}
    77  	}
    78  }
    79  
    80  type trInFlow struct {
    81  	limit               uint32
    82  	unacked             uint32
    83  	effectiveWindowSize uint32
    84  }
    85  
    86  func (f *trInFlow) newLimit(n uint32) uint32 {
    87  	d := n - f.limit
    88  	f.limit = n
    89  	f.updateEffectiveWindowSize()
    90  	return d
    91  }
    92  
    93  func (f *trInFlow) onData(n uint32) uint32 {
    94  	f.unacked += n
    95  	if f.unacked >= f.limit/4 {
    96  		w := f.unacked
    97  		f.unacked = 0
    98  		f.updateEffectiveWindowSize()
    99  		return w
   100  	}
   101  	f.updateEffectiveWindowSize()
   102  	return 0
   103  }
   104  
   105  func (f *trInFlow) reset() uint32 {
   106  	w := f.unacked
   107  	f.unacked = 0
   108  	f.updateEffectiveWindowSize()
   109  	return w
   110  }
   111  
   112  func (f *trInFlow) updateEffectiveWindowSize() {
   113  	atomic.StoreUint32(&f.effectiveWindowSize, f.limit-f.unacked)
   114  }
   115  
   116  func (f *trInFlow) getSize() uint32 {
   117  	return atomic.LoadUint32(&f.effectiveWindowSize)
   118  }
   119  
   120  // TODO(mmukhi): Simplify this code.
   121  // inFlow deals with inbound flow control
   122  type inFlow struct {
   123  	mu sync.Mutex
   124  	// The inbound flow control limit for pending data.
   125  	limit uint32
   126  	// pendingData is the overall data which have been received but not been
   127  	// consumed by applications.
   128  	pendingData uint32
   129  	// The amount of data the application has consumed but grpc has not sent
   130  	// window update for them. Used to reduce window update frequency.
   131  	pendingUpdate uint32
   132  	// delta is the extra window update given by receiver when an application
   133  	// is reading data bigger in size than the inFlow limit.
   134  	delta uint32
   135  }
   136  
   137  // newLimit updates the inflow window to a new value n.
   138  // It assumes that n is always greater than the old limit.
   139  func (f *inFlow) newLimit(n uint32) {
   140  	f.mu.Lock()
   141  	f.limit = n
   142  	f.mu.Unlock()
   143  }
   144  
   145  func (f *inFlow) maybeAdjust(n uint32) uint32 {
   146  	if n > uint32(math.MaxInt32) {
   147  		n = uint32(math.MaxInt32)
   148  	}
   149  	f.mu.Lock()
   150  	defer f.mu.Unlock()
   151  	// estSenderQuota is the receiver's view of the maximum number of bytes the sender
   152  	// can send without a window update.
   153  	estSenderQuota := int32(f.limit - (f.pendingData + f.pendingUpdate))
   154  	// estUntransmittedData is the maximum number of bytes the sends might not have put
   155  	// on the wire yet. A value of 0 or less means that we have already received all or
   156  	// more bytes than the application is requesting to read.
   157  	estUntransmittedData := int32(n - f.pendingData) // Casting into int32 since it could be negative.
   158  	// This implies that unless we send a window update, the sender won't be able to send all the bytes
   159  	// for this message. Therefore we must send an update over the limit since there's an active read
   160  	// request from the application.
   161  	if estUntransmittedData > estSenderQuota {
   162  		// Sender's window shouldn't go more than 2^31 - 1 as specified in the HTTP spec.
   163  		if f.limit+n > maxWindowSize {
   164  			f.delta = maxWindowSize - f.limit
   165  		} else {
   166  			// Send a window update for the whole message and not just the difference between
   167  			// estUntransmittedData and estSenderQuota. This will be helpful in case the message
   168  			// is padded; We will fallback on the current available window(at least a 1/4th of the limit).
   169  			f.delta = n
   170  		}
   171  		return f.delta
   172  	}
   173  	return 0
   174  }
   175  
   176  // onData is invoked when some data frame is received. It updates pendingData.
   177  func (f *inFlow) onData(n uint32) error {
   178  	f.mu.Lock()
   179  	f.pendingData += n
   180  	if f.pendingData+f.pendingUpdate > f.limit+f.delta {
   181  		limit := f.limit
   182  		rcvd := f.pendingData + f.pendingUpdate
   183  		f.mu.Unlock()
   184  		return fmt.Errorf("received %d-bytes data exceeding the limit %d bytes", rcvd, limit)
   185  	}
   186  	f.mu.Unlock()
   187  	return nil
   188  }
   189  
   190  // onRead is invoked when the application reads the data. It returns the window size
   191  // to be sent to the peer.
   192  func (f *inFlow) onRead(n uint32) uint32 {
   193  	f.mu.Lock()
   194  	if f.pendingData == 0 {
   195  		f.mu.Unlock()
   196  		return 0
   197  	}
   198  	f.pendingData -= n
   199  	if n > f.delta {
   200  		n -= f.delta
   201  		f.delta = 0
   202  	} else {
   203  		f.delta -= n
   204  		n = 0
   205  	}
   206  	f.pendingUpdate += n
   207  	if f.pendingUpdate >= f.limit/4 {
   208  		wu := f.pendingUpdate
   209  		f.pendingUpdate = 0
   210  		f.mu.Unlock()
   211  		return wu
   212  	}
   213  	f.mu.Unlock()
   214  	return 0
   215  }
   216
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