Monday, February 1, 2016

ZeroMQ - batching

ØMQ tries to deliver consistently low latencies combined with high throughput using the following strategy: when message flow is sparse and doesn't exceed the network stack's bandwidth, ØMQ turns all the batching off to improve latency. The trade-off here is somewhat higher CPU usage—we still have to traverse the stack frequently. However, that isn't considered to be a problem in most cases.

When the message rate exceeds the bandwidth of the network stack, in such situations it makes sense to start batching aggressively. There's nothing to lose as the latencies are already high anyway. On the other hand, aggressive batching improves throughput and can empty the queue of pending messages—which in turn means the latency will gradually drop as the queueing delay decreases. Once there are no outstanding messages in the queue, the batching can be turned off to improve the latency even further.

One additional observation is that the batching should only be done on the topmost level. If the messages are batched there, the lower layers have nothing to batch anyway, and so all the batching algorithms underneath do nothing except introduce additional latency.

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