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Joel on Software


by Joel Spolsky
Tuesday, January 28, 2003

Binary Search Debugging

Something we had done since the last release of CityDesk somehow caused our publish times to increase by about 100%; on a particular large site we use for stress testing it had gone from about a minute to about two minutes.

The first thing I tried was a profiler: Compuware DevPartner Studio. Indeed this showed me where a lot of bottlenecks are; that data will be useful to speed up our publish times even more, but I really wanted to find the specific bug that I thought we had introduced which was slowing us down.

The next thing I tried was a method I learned from Gabi at Juno: the old binary search method. Before we started work on this release, publishing took 1'04". Today it takes 1'57". So I started checking out old versions of the source from CVS by date, rebuilding, and timing how long publishing took with each day's build. Here's what I found:

As of May 1: 1'57"
As of April 1: 1'05"
As of April 15: 1'05"
As of April 22: 1'06"
As of April 26: 1'58"
As of April 24: 1'05"
As of April 25: 1'05"

Aha! Now all I had to do was run WinDiff to compare the source tree from April 25th and April 26th, and I discovered four things that were changed that day, one of which was a function that DevPartner had told me was kind of slow, anyway. Within minutes I found the culprit -- that function was originally written to cache its results because it's often called with the same inputs, and I had inadvertently changed the cache key in one place and not another, so we were getting 100% misses instead of 99% hits. Solved! Total elapsed time to find this bug: about an hour. If your source code is much bigger than CityDesk, builds and checkouts may be slow. This is as good a reason as any to keep all your old daily builds around.

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