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Wed, 04 Jul 2018

Jackson and Gregg on optimization

Today Brendan Gregg's blog has an article Evaluating the Evaluation: Benchmarking Checklist that begins:

A co-worker introduced me to Craig Hanson and Pat Crain's performance mantras, which neatly summarize much of what we do in performance analysis and tuning. They are:

Performance mantras

  1. Don't do it
  2. Do it, but don't do it again
  3. Do it less
  4. Do it later
  5. Do it when they're not looking
  6. Do it concurrently
  7. Do it cheaper

I found this striking because I took it to be an obvious reference Michael A. Jackson's advice in his brilliant 1975 book Principles of Program Design. Jackson said:

We follow two rules in the matter of optimization:

Rule 1: Don't do it.
Rule 2 (for experts only). Don't do it yet.

The intent of the two passages is completely different. Hanson and Crain are offering advice about what to optimize. “Don't do it” means that to make a program run faster, eliminate some of the things it does. “Do it, but don't do it again” means that to make a program run faster, have it avoid repeating work it has already done, say by caching results. And so on.

Jackson's advice is of a very different nature. It is only indirectly about improving the program's behavior. Instead it is addressing the programmer's behavior: stop trying to optimize all the damn time! It is not about what to optimize but whether, and Jackson says that to a first approximation, the answer is no.

Here are Jackson's rules with more complete context. The quotation is from the preface (page vii) and is discussing the style of the examples in his book:

Above all, optimization is avoided. We follow two rules in the matter of optimization:

Rule 1. Don't do it.
Rule 2 (for experts only). Don't do it yet — that is, not until you have a perfectly clear and unoptimized solution.

Most programmers do too much optimization, and virtually all do it too early. This book tries to act as an antidote. Of course, there are systems which must be highly optimized if they are to be economically useful, and Chapter 12 discusses some relevant techniques. But two points should always be remembered: first, optimization makes a system less reliable and harder to maintain, and therefore more expensive to build and operate; second, because optimization obscures structure it is difficult to improve the efficiency of a system which is already partly optimized.

Here's some code I dealt with this month:

    my $emailclass = $args->{emailclass};
    if (!$emailclass && $args->{emailclass_name} ) {
      # do some caching so if we're called on the same object over and over we don't have to do another find.
      my $last_emailclass = $self->{__LAST_EMAILCLASS__};
      if ( $last_emailclass && $last_emailclass->{name} eq $args->{emailclass_name} ) {
        $emailclass = $last_emailclass->{emailclass};
      } else {
        $emailclass = $self->schema->resultset('EmailClass')
        $self->{__LAST_EMAILCLASS__} = {
                                        name => $args->{emailclass_name},
                                        emailclass => $emailclass,

Holy cow, this is wrong in so many ways. 8 lines of this mess, for what? To cache a single database lookup (the ->find_by_name call), in a single object, if it happens to be looking for the same name as last time. If caching was actually wanted, it should have been addressed in the ->find_by_name call, which could do the caching more generally, and which has some hope of knowing something about when the cache entries should be expired. Even stipulating that caching was wanted and for some reason should have been put here, why such an elaborate mechanism, all to cache just the last lookup? It could have been:

    $emailclass = $self->emailclass_for_name($args->{emailclass_name});

    sub emailclass_for_name {
      my ($self, $name) = @_;
      $self->{emailclass}{$name} //=
      return $self->{emailclass}{$name};

I was able to do a bit better than this, and replaced the code with:

    $emailclass = $self->schema->resultset('EmailClass')

My first thought was that the original caching code had been written by a very inexperienced programmer, someone who with more maturity might learn to do their job with less wasted effort. I was wrong; it had been written by a senior developer, someone who with more maturity might learn to do their job with less wasted effort.

The tragedy did not end there. Two years after the original code was written a more junior programmer duplicated the same unnecessary code elsewhere in the same module, saying:

I figured they must have had a reason to do it that way…

Thus is the iniquity of the fathers visited on the children.

In a nearby piece of code, an object A, on the first call to a certain method, constructed object B and cached it:

    base_path => ...
    schema    => $self->schema,
    retry     => ...,

Then on subsequent calls, it reused B from the cache.

But the cache was shared among many instances of A, not all of which had the same ->schema member. So some of those instances of A would ask B a question and get the answer from the wrong database. A co-worker spent hours and hours in the middle of the night last month tracking this down. Again, the cache was not only broken but completely unnecesary. What was being saved? A single object construction, probably a few hundred bytes and a few hundred microseconds at most. And again, the code was perpetrated by a senior developer who should have known better. My co-worker replaced 13 lines of broken code with four that worked.

Brendan Gregg is unusually clever, and an exceptional case. Most programmers are not Brendan Gregg, and should take Jackson's advice and stop trying to be so clever all the time.

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