question about puma architecture

[izenk]

Hello.
Have some unclear moments after reading puma architecture docs, exact section:

• Upon startup, Puma listens on a TCP or UNIX socket.
  • The backlog of this socket is configured with a default of 1024, but the actual backlog value is capped by the net.core.somaxconn sysctl value. The backlog determines the size of the queue for unaccepted connections. If the backlog is full, the operating system is not accepting new connections.
  • This socket backlog is distinct from the backlog of work as reported by Puma.stats or the control server. The backlog that Puma.stats refers to represents the number of connections in the process' todo set waiting for a thread from the ThreadPool. - so, todo set is some kind of internal puma queue where buffered (mean accepted requests) are waiting to be pickup-ed by thread?
• By default, a single, separate thread (created by the Reactor class) reads and buffers requests from the socket.
  • (!) When at least one worker thread is available for work, the reactor thread listens to the socket and accepts a request (if one is waiting). - does this mean that request from socket will be accepted only if worker thread is available? But If so, in situation when all threads are serving requests (so no threads are available) it means that todo set will always be empty, because no requests will be picked-up from socket backlog, correct? I don't understand how puma stat may show something like "backlog": 5, because it means that requests have been put into todo set, while threads were busy serving requests which contradicts the statement marked with (!) above.

[stevenharman]

so, todo set is some kind of internal puma queue where buffered (mean accepted requests) are waiting to be pickup-ed by thread?

Yes, that's right. At least, that's true so long as the queue_requests option is true (which is the default).

As for the question about how/when the "todo" queue is filled from the Socket, I'm also curious about that. Is the "todo" queue bounded somehow? What controls how big it can get?