[Buildbot-devel] Why slaves locked by a builder lock are considered as available by this builder

[Dustin J. Mitchell]

So, the problem with slavelocks is that they are acquired *after* a
build is begun, and they simply block the build if the lock is already
held by another build, rather than re-trying to distribute the
buildrequest, possibly to another slave.  SlaveLocks are good for
locking around a particular *step* -- say, around a step that uses the
database instance hosted on the slave to run some tests -- rather than
for the entire build.

Generally, the best way to limit parallelism is with the maxBuilds
slave argument, as this is checked before a build is started.

Dustin