Performance and Evaluation of Lisp Systems - Dreamsongs

§ 3.5 Takr 111
where CPU time is the EBOX time (no memory reference time included), elapsed
time is real time, wholine time is EBOX + MBOX (memory reference) times, GC
time is garbage collector time, and the load averages are given before and after
the timing run. All times are in seconds, and the load average is the exponentially
weighted, over time, average of the number of jobs in all runnable queues. With
no load, wholine and elapsed times are the same.
On TAKR with no load the result on SAIL (KL-10A) was
CPU time = 0.602
elapsed time = 1.02
wholine time = 1.02
gc time = 0.0
load average before = 0.27
load average after = 0.28
which shows a 36% degradation. The question is how well these 100 functions
destroy the effect of the cache on SAIL. The answer is that it does not destroy the
effect very much. This makes sense because the total number of instructions for
100 copies of the function is about 3800, and the cache holds about 2000 words.
Here is the result for the single function TAK ′ run on SAIL with the cache shut off:
CPU time = 0.6
elapsed time = 6.95
wholine time = 6.9
gc time = 0.0
load average before = 0.036
load average after = 0.084
which shows a factor of 9.2 degradation. The 100-function version ran in the same
time within a few percent.
Hence, in order to destroy the effect of a cache, one must increase the size of
the code significantly beyond the size of the cache. Also, the distribution of the
locus of control must be roughly uniform or random.
More important to most implementations, though, is that the tail recursion is
no longer guaranteed to be a call from a function to itself, and many implementa-
tions do not do tail recursion removal when the call is not to that same function.
That is, often a compiler will implement tail recursion removal by transforming a
function that calls itself to one that does a GO to the head of the function. Of
course, the function has to be converted to a PROG equivalent of itself as well.