If the Lisp system supports multithreading, SLIME spawns a new thread for each request, e.g., C-x C-e creates a new thread to evaluate the expression. An exception to this rule are requests from the REPL: all commands entered in the REPL buffer are evaluated in a dedicated REPL thread.
Some complications arise with multithreading and special variables.
Non-global special bindings are thread-local, e.g., changing the value
of a let bound special variable in one thread has no effect on the
binding of the variables with the same name in other threads. This
makes it sometimes difficult to change the printer or reader behaviour
for new threads. The variable
swank:*default-worker-thread-bindings* was introduced for such
situations: instead of modifying the global value of a variable, add a
binding the swank:*default-worker-thread-bindings*. E.g., with
the following code, new threads will read floating point values as
doubles by default:
(push '(*read-default-float-format* . double-float)
swank:*default-worker-thread-bindings*).