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Multi-threaded (MT) servers, depicted in Figure 3, employ multiple independent threads of control operating within a single shared address space. Each thread performs all the steps associated with one HTTP request before accepting a new request, similar to the MP model's use of a process.
The primary difference between the MP and the MT architecture, however, is that all threads can share global variables. The use of a single shared address space lends itself easily to optimizations that rely on shared state. However, the threads must use some form of synchronization to control access to the shared data.
The MT model requires that the operating system provides support for kernel threads. That is, when one thread blocks on an I/O operation, other runnable threads within the same address space must remain eligible for execution. Some operating systems (e.g., FreeBSD 2.2.6) provide only user-level thread libraries without kernel support. Such systems cannot effectively support MT servers.