A few of Kernighan and Van Wyk's conclusions about the performance of Scheme resulted from their choice of an extremely slow interpreter as their canonical implementation of Scheme. What would they have concluded if they had benchmarked a compiler instead?

One of Kernighan and Van Wyk's conclusions was that purely functional tail recursion is no faster in Scheme than an imperative style that performs assignments to global variables:

we wrote two Scheme versions of the timing loop, string construction, and word count, one using the tail-recursion favored by Scheme aficionados, the other using the barbarian import set!; the difference in performance between the two versions never exceeded ten percent, and did not consistently favor one version over the other.

In Larceny, however, the tail recursive version of the timing loop is over three times as fast as the version using set!. The tail recursive version is only twenty percent faster in Gambit-C (with fixnum arithmetic) and in Chez Scheme, but that is still a dramatic difference.

This difference does not exist with the interpreted implementations, nor with the other two benchmarks mentioned above, because the overhead of interpretation, string manipulation, or character i/o are so dominant that they mask the expense of assignments.

It is also worth noting that Scheme's do construct, which Kernighan and Van Wyk characterized as an iteration construct "for unreconstructed imperative-style programmers", is not imperative at all: The loop variables are updated by binding, as in any other tail recursion, not by assignment. In compiled implementations of Scheme, the do construct has exactly the same overhead as the tail recursion for which it is an abbreviation.

Concerning Scheme's poor i/o performance, Kernighan and Van Wyk concluded that

The enormous Scheme runtimes appear to be caused by the lack of facilities for buffered input and output.

This is true, although Chez Scheme shows that Scheme can get to within a factor of 2 of C even on i/o benchmarks. It is almost entirely a matter of implementation effort, and I hope the Kernighan and Van Wyk i/o benchmarks will encourage Scheme implementors to improve their i/o performance. (That's easy for me to say, because we've already made these improvements in a more recent version of Larceny.)

Scheme appears to be very roughly comparable with Tcl...various compilation techniques improve this behavior.

Last updated 2 December 1997