zeromq batching

"* The graph in the test results shows that ZeroMQ is slower than TCP/IP. What's the point then?

Obviously, you would expect system working on top of TCP to have higher latencies than TCP. Anything else would be - simply speaking - supernatural. However, throughput is a different matter. ZeroMQ gets you more throughput than TCP has using intelligent batching algorithms. Moreover ZeroMQ delivers value-add over the TCP. Asynchronicity, message queueing, routing based on business logic, multicast etc.

* How come ZeroMQ has higher throughput than TCP although it's built on top of TCP?

Avoiding redundant networking stack traversals can improve throughput significantly. In other words, sending two messages down the networking stack in one go takes much less time then sending each of them separately.This technique is known as message batching.

* When sending messages in batches you have to wait for the last one to send the whole batch. This would make the latency of the first message in the batch much worse, wouldn't it?

ZeroMQ batches messages in opportunistic manner. Rather than waiting for a predefined number of messages and/or predefined time interval, it sends all the messages available at the moment in one go. Imagine the network interface card is busy sending data. Once it is ready to send more data it asks ZeroMQ for new messages. ZeroMQ sends all the messages available at the moment. Does it harm the latency of the first message in the batch? No. The message won't be sent earlier anyway because the networking card was busy. On the contrary, latency of subsequent messages will be improved because sending single batch to the card is faster then sending lot of small messages. On the other hand, if network card isn't busy, the message is sent straight away without waiting for following messages. Thus it'll have the best possible latency."