Anecdote or data point

I saw that there was some argument over how much slower slot access is than struct access, so I just decided to measure it naively. I made a two slot sruct and a CLOS version of a CONS cell with car and cdr slots and I ran LTAK using regular lists, `lists' made from CLOS conses, and `lists' made from structs. Here are the results:

D:\repositories\clos-benchmark>sbcl --script run-benchmarks.lisp
Benchmark: ltak over native cons cells, CLOS my-cons nodes, and my-cons-struct nodes
Inputs: x=15 y=9 z=4 repeats=35

Scenario                   min-ms     mean-ms      max-ms      ratio
--------------------------------------------------------------------
native standard               0.129      0.146      0.186
clos standard                 1.346      1.365      1.475       9.37x
struct standard               0.172      0.175      0.179       1.20x
native optimized              0.068      0.069      0.073
clos optimized                0.411      0.414      0.419       6.04x
struct optimized              0.068      0.069      0.073       1.01x

In this naive use case, structs are same as native cons cells, but CLOS objects are one ninth the speed of a struct or cons cell if you just use it unoptimized, and one sixth the speed if optimizations are turned on.

But the CLOS instance is more functional than the cons cell in mimics. For instance, I could add a slot to the class and all the instances would be lazily updated with the new slot. I can also subclass the CLOS class and the selector functions will continue to work. Finally, I can redefine the CLOS closs while I'm developing it and all the instances will be uppdated. THe machinery to keep all this running is costing us our factor of 9.

But this might be worth the cost if we are running on a network where the bulk of the time will be transmitting the answer down the pipe once it is computed. Taking a few extra milliseconds to compute the answer might be worth the convenience features of CLOS.