kafka producer的batch.size和linger.ms

1.问题

batch.size和linger.ms是对kafka producer性能影响比较大的两个参数。batch.size是producer批量发送的基本单位，默认是16384Bytes，即16kB；lingger.ms是sender线程在检查batch是否ready时候，判断有没有过期的参数，默认大小是0ms。

那么producer是按照batch.size大小批量发送消息呢，还是按照linger.ms的时间间隔批量发送消息呢？这里先说结论：其实满足batch.size和ling.ms之一，producer便开始发送消息。

2.源码分析

首先sender线程主要代码如下,我们主要关心sender线程阻塞的情况:

void run(long now) {
        Cluster cluster = metadata.fetch();

        // result.nextReadyCheckDelayMs表示下次检查是否ready的时间，也是//selecotr会阻塞的时间
        RecordAccumulator.ReadyCheckResult result = this.accumulator.ready(cluster, now);

        if (result.unknownLeadersExist)
            this.metadata.requestUpdate();

        Iterator<Node> iter = result.readyNodes.iterator();

        long notReadyTimeout = Long.MAX_VALUE;
        while (iter.hasNext()) {
            Node node = iter.next();
            if (!this.client.ready(node, now)) {
                iter.remove();
                notReadyTimeout = Math.min(notReadyTimeout, this.client.connectionDelay(node, now));
            }
        }

        Map<Integer, List<RecordBatch>> batches = this.accumulator.drain(cluster,
                                                                         result.readyNodes,
                                                                         this.maxRequestSize,
                                                                         now);
        if (guaranteeMessageOrder) {
            for (List<RecordBatch> batchList : batches.values()) {
                for (RecordBatch batch : batchList)
                    this.accumulator.mutePartition(batch.topicPartition);
            }
        }

        List<RecordBatch> expiredBatches = this.accumulator.abortExpiredBatches(this.requestTimeout, now);
    
        for (RecordBatch expiredBatch : expiredBatches)
            this.sensors.recordErrors(expiredBatch.topicPartition.topic(), expiredBatch.recordCount);

        sensors.updateProduceRequestMetrics(batches);
        List<ClientRequest> requests = createProduceRequests(batches, now);

        // 暂且只关心result.nextReadyCheckDelayMs
        long pollTimeout = Math.min(result.nextReadyCheckDelayMs, notReadyTimeout);
        if (result.readyNodes.size() > 0) {
            log.trace("Nodes with data ready to send: {}", result.readyNodes);
            log.trace("Created {} produce requests: {}", requests.size(), requests);
            pollTimeout = 0;
        }
        for (ClientRequest request : requests)
            client.send(request, now);

        // poll最终会调用selector,pollTimeout也就是selector阻塞的时间
        this.client.poll(pollTimeout, now);
    }

selector

private int select(long ms) throws IOException {
        if (ms < 0L)
            throw new IllegalArgumentException("timeout should be >= 0");

        if (ms == 0L)
            return this.nioSelector.selectNow();
        else
            return this.nioSelector.select(ms);
    }

我们可以从实例化一个新的KafkaProducer开始分析（还没有调用send方法），在sender线程调用accumulator#ready(..)时候，会返回result，其中包含selector可能要阻塞的时间。由于还没有调用send方法，所以Deque<RecordBatch>为空，所以result中包含的nextReadyCheckDelayMs也是最大值，这个时候selector会一直阻塞。

public ReadyCheckResult ready(Cluster cluster, long nowMs) {
        Set<Node> readyNodes = new HashSet<Node>();
         // 初始化为最大值
        long nextReadyCheckDelayMs = Long.MAX_VALUE;
        boolean unknownLeadersExist = false;

        boolean exhausted = this.free.queued() > 0;
        for (Map.Entry<TopicPartition, Deque<RecordBatch>> entry : this.batches.entrySet()) {
            TopicPartition part = entry.getKey();
            Deque<RecordBatch> deque = entry.getValue();

            Node leader = cluster.leaderFor(part);
            if (leader == null) {
                unknownLeadersExist = true;
            } else if (!readyNodes.contains(leader) && !muted.contains(part)) {
                synchronized (deque) {
                    RecordBatch batch = deque.peekFirst();
                    if (batch != null) {
                        boolean backingOff = batch.attempts > 0 && batch.lastAttemptMs + retryBackoffMs > nowMs;
                        long waitedTimeMs = nowMs - batch.lastAttemptMs;
                        long timeToWaitMs = backingOff ? retryBackoffMs : lingerMs;

                        // 和linger.ms有关
                        long timeLeftMs = Math.max(timeToWaitMs - waitedTimeMs, 0);
                        boolean full = deque.size() > 1 || batch.records.isFull();
                        boolean expired = waitedTimeMs >= timeToWaitMs;
                        boolean sendable = full || expired || exhausted || closed || flushInProgress();
                        if (sendable && !backingOff) {
                            readyNodes.add(leader);
                        } else {
                            nextReadyCheckDelayMs = Math.min(timeLeftMs, nextReadyCheckDelayMs);
                        }
                    }
                }
            }
        }

        return new ReadyCheckResult(readyNodes, nextReadyCheckDelayMs, unknownLeadersExist);
    }

然后我们调用send方法往内存中放入了一条数据，由于是新建的一个RecordBatch，所以会唤醒sender线程
KafkaProducer#doSend(...)

if (result.batchIsFull || result.newBatchCreated) {
                log.trace("Waking up the sender since topic {} partition {} is either full or getting a new batch", record.topic(), partition);
                this.sender.wakeup();
            }

这个时候会唤醒阻塞在selector#select(..)的sender线程，sender线程又运行到accumulator#ready(..)，由于Deque<RecordBatch>有值，所以返回的result包含的nextReadyCheckDelayMs不再是最大值，而是和linger.ms有关的值。也就是时候selector会z最多阻塞lingger.ms后就返回，然后再次轮询。

也就是说当Deque<RecordBatch>不为空的时候，sender线程会最多阻塞linger.ms时间；Deque<RecordBatch>为空的时候，sender线程会阻塞Long.MAX_VALUE时间；一旦调用了KafkaProduer#send(..)将消息放到内存中，新建了个RecordBatch，则会将sender线wakeup。

另外从上面的代码，即KafkaProducer#doSend(...)中也可以看到，如果有一个RecordBatch满了，也会调用Sender#wakeup(..)，所以综上所述：只要满足linger.ms和batch.size满了就会激活sender线程来发送消息。