Performance and Evaluation of Lisp Systems - Dreamsongs

§ 1.1 Levels of Lisp System Architecture 3
is not necessarily accurate when comparing implementations. For example, pro-
grammers, knowing the performance profile of their machine and implementation,
will typically bias their style of programming on that piece of code. Hence, had an
expert on another system attempted to program the same algorithms, a different
program might have resulted.
1.1.1 Hardware Level
At the lowest level, things like the machine clock speed and memory band-
width affect the speed of a Lisp implementation. One might expect a CPU with a
basic clock rate of 50 nanoseconds to run a Lisp system faster than the same ar-
chitecture with a clock rate of 500 nanoseconds. This, however, is not necessarily
true, since a slow or small memory can cause delays in instruction and operand
fetch.
Several hardware facilities complicate the understanding of basic system per-
formance, especially on microcoded machines: the memory system, the instruction
buffering and decoding, and the size of data paths. The most important of these
facilities will be described in the rest of this section.
Cache memory is an important and difficult-to-quantify determiner of per-
formance. It is designed to improve the speed of programs that demonstrate a lot
of locality 2 by supplying a small high-speed memory that is used in conjunction
with a larger, but slower (and less expensive) main memory. An alternative to a
cache is a stack buffer, which keeps some number of the top elements of the stack
in a circular queue of relatively high-speed memory. The Symbolics 3600 has such
a PDL buffer.
Getting a quantitative estimate of the performance improvement yielded by
a cache memory can best be done by measurement and benchmarking. Lisp has
less locality than many other programming languages, so that a small benchmark
may fail to accurately measure the total performance by failing to demonstrate
‘normal’ locality. Hence, one would expect the small-benchmark methodology to
tend to result in optimistic measurements, since small programs have atypically
higher locality than large Lisp programs.
2 Locality is the extent to which the locus of memory references—both instruction fetches
and data references—span a ‘small’ number of memory cells ‘most’ of the time.