The prefetch size dictates how many messages will be held in RAM on the client so if your RAM is limited you may want to set a low value such as 1 or 10 etc.

What is QOS in RabbitMQ?

Quality of Service (QOS) AMQP Channels contain a property called QOS. The value of this property determines the manner in which AMQP Consumers read messages from Queues. A QOS value of “1” ensures that only a single message at a time will be de-queued.

What does Unacked mean?

Unacked means that the consumer has promised to process them but has not acknowledged that they are processed. When the consumer crashed the queue knows which messages are to be delivered again when the consumer comes online. When you have multiple consumers the messages are distributed among them.