AutoSTR | Efficient Backbone Search for Scene Text Recognition | Computer Vision library

In Cap'n Proto, you should not reuse a MessageBuilder for multiple messages. The way you've written your code, every iteration of your loop will make the message bigger, because you're actually adding on to the existing message rather than starting a new one. To avoid memory allocation with each iteration, you should pass a scratch buffer to MallocMessageBuilder's constructor. The scratch buffer can be allocated once outside the loop, but you need to create a new MallocMessageBuilder each time around the loop. (Of course, most people don't bother with scratch buffers and just let MallocMessageBuilder do its own allocation, but if you choose that path in this benchmark, then you should also change the Protobuf benchmark to create a new message object for every iteration rather than reusing a single object.)

Additionally, your Cap'n Proto code is using capnp::messageToFlatArray(), which allocates a whole new buffer to put the message into and copies the entire message over. This is not the most efficient way to use Cap'n Proto. Normally, if you were writing the message to a file or socket, you would write directly from the message's original backing buffer(s) without making this copy. Try doing this instead: