Most of Prolog's memory usage consists of pointers. This indicates
the primary drawback: Prolog memory usage almost doubles when using the
64-bit addressing model. Using more memory means copying more data
between CPU and main memory, slowing down the system.
What then are the advantages? First of all, SWI-Prolog's addressing
of the Prolog stacks does not cover the whole address space due to the
use of type tag bits and garbage collection flags. On
32-bit hardware the stacks are limited to 128 MB each. This tends
to be too low for demanding applications on modern hardware. On 64-bit
hardware the limit is 2^32 times higher, exceeding the
addressing capabilities of today's CPUs and operating systems. This
implies Prolog can be started with stack sizes that use the full
capabilities of your hardware.
Multi-threaded applications profit much more because every thread has
its own set of stacks. The Prolog stacks start small and are dynamically
expanded (see section
2.19.1). The C stack is also dynamically expanded, but the maximum
size is reserved when a thread is started. Using 100 threads at
the maximum default C stack of 8Mb (Linux) costs 800Mb virtual memory!41C-recursion
over Prolog data structures is removed from most of SWI-Prolog. When
removed from all predicates it will often be possible to use lower
limits in threads. See http://www.swi-prolog.org/Devel/CStack.html
The implications
of theoretical performance loss due to increased memory bandwidth
implied by exchanging wider pointers depend on the design of the
hardware. We only have data for the popular IA32 vs. AMD64
architectures. Here, it appears that the loss is compensated for by an
instruction set that has been optimized for modern programming. In
particular, the AMD64 has more registers and the relative addressing
capabilities have been improved. Where we see a 10% performance
degradation when placing the SWI-Prolog kernel in a Unix shared object,
we cannot find a measurable difference on AMD64.
