Most of today's 64-bit platforms are
capable of running both 32-bit and 64-bit applications. This asks for
some clarifications on the advantages and drawbacks of 64-bit addressing
for (SWI-)Prolog.
SWI-Prolog can be compiled for a
32- or 64-bit address space on any system with a suitable C compiler.
Pointer arithmetic is based on the type (u)intptr_t from stdint.h,
with suitable emulation on MS-Windows.
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.
For those cases where we can choose between 32 and 64 bits, either
because the hardware and OS support both or because we can still choose
the hardware and OS, we give guidelines for this decision.
First of all, if SWI-Prolog needs to be linked against 32- or 64-bit
native libraries, there is no choice as it is not possible to link 32-
and 64-bit code into a single executable. Only if all required libraries
are available in both sizes and there is no clear reason to use either
do the different characteristics of Prolog become important.
Prolog applications that require more than the 128 MB stack
limit provided in 32-bit addressing mode must use the 64-bit edition.
Note however that the limits must be doubled to accommodate the same
Prolog application.
If the system is tight on physical memory, 32-bit Prolog has the
clear advantage of using only slightly more than half of the memory of
64-bit Prolog. This argument applies as long as the application fits in
the
virtual address space of the machine. The virtual address space
of 32-bit hardware is 4GB, but in many cases the operating system
provides less to user applications.
The only standard SWI-Prolog
library adding significantly to this calculation is the RDF database
provided by the semweb package. It uses approximately 80 bytes
per triple on 32-bit hardware and 150 bytes on 64-bit hardware. Details
depend on how many different resources and literals appear in the
dataset as well as desired additional literal indexes.
Summarizing, if applications are small enough to fit comfortably in
virtual and physical memory, simply take the model used by most of the
applications on the OS. If applications require more than 128 MB
per stack, use the 64-bit edition. If applications approach the size of
physical memory, fit in the 128 MB stack limit and fit in virtual
memory, the 32-bit version has clear advantages. For demanding
applications on 64-bit hardware with more than about 6GB physical memory
the 64-bit model is the model of choice.
