We investigate high-performance threading architectures for I/O intensive multi-threaded servers. We study thread architectures from two angles: (1) number of user threads per kernel thread, and (2) use of synchronous I/O vs. asynchronous I/O. We underline the shortcomings of 1-to-1threads with synchronous I/O, N-to-1 threads with asynchronous I/O, and N-to-M threads with synchronous I/O with respect to server performance We propose N-to-M threads with asynchronous I/O, a novel and previously unexplored thread model, for such servers. We explain the architectural benefits of this thread model over the above mentioned architectures. We have designed and implemented ServLib, a thread library, modeled in this architecture. We optimize ServLib to reduce context switches for large I/O transfers. ServLib exports standard POSIX threads (pthreads) API and can be slipped transparently beneath any multi-threaded application using such API. We have evaluated ServLib with two applications, the Apache multi-threaded web server, and the MySQL multi-threaded database server. Our results show that for synthetic and real workloads Apache with ServLib registers a performance improvement of 10-25%over Apache with 1-to-1 thread and N-to-1 thread libraries. For TPC-W workload, ServLib improves the performance of MySQL by 10-17% over 1-to-1 and N-to-1thread libraries.