Tomcat connector architecture, thread pools and async servlets

• acceptorThread(s) : This is a single or at the most 2 threads (as mentioned in the doc) which is responsible only for accepting in-coming connections. This can be configured using acceptorThreadCount, and it's suggested that more than two can be used for a multi-cpu machine -

  why is this ?

Accepting connections is a very low cost operation so it just doesn't make sense to dedicate more than one thread to this task.

Does this imply that the number of simultaneous open connections scales with the number of cpus versus the number of open file descriptors allowed on the server system ?

No it doesn't because it's a very low cost operation in terms of CPU. As for file descriptors, each accepted connection is going to use a file descriptor, so the maxim number of the connections the server can accept is limited by the number of available file descriptors.

• maxConnections(s) :

  What is the relation between this setting and acceptCount and the number of open file descriptors on the system.

maxConnections cannot be higher the number of open file descriptors on the system. Keep in mind that other processes use file descriptors as well, so may want to be conservative with maxConnections in regards to the available file descriptors, let's say maxConnections < file descriptors / 2.

Why is the default value for this so much higher for the NIO connector ( 10000 ) than for the BIO ( = maxThreads ) ?

This is because in NIO a single handles all IO, and in BIO the server need to create/use a separate thread-per connection.

• acceptCount : This is the queue for requests when all request processing threads are busy.

  When requests are put into this queue, is a file descriptor assigned to it as well ?

Yes, that's correct the connection request is accepted but the server not yet ready to being serving requests, so the connection is placed in the queue. This is done to prevent TCP/stack timing out connection requests which may look like a server down from the client point of view. In other words, server says 'I'm here and will process your request once I have resources to do it'.

Or is it only when a request is being actively processed, does it consume a file descriptor ?

Nope.

Hope this helps.

Regards,

Slava Imeshev