[Buildbot] #2895: Builds take a long time to get started on latent buildslaves

[Buildbot trac]

#2895: Builds take a long time to get started on latent buildslaves
------------------------+------------------------
Reporter:  agateau      |       Owner:
    Type:  enhancement  |      Status:  new
Priority:  major        |   Milestone:  undecided
 Version:  0.8.6p1      |  Resolution:
Keywords:  ec2          |
------------------------+------------------------

Comment (by agateau):

 Replying to [comment:8 dustin]:
 > So you had a `print` or `log.msg` both before and after that statement,
 and the time difference between those was four minutes?

 (I should have kept the log-riddled files, this is from memory)

 I added a `log.msg` before acquiring `pending_builders_lock` in
 `BotMaster.maybeStartBuildsOn` and one after releasing it at the end of
 the function.

 With this on, I noticed that the entire time between the build being added
 and the build actually starting, the log after unlocking was called at a
 rate of roughly 3 or 4 times per second (again, from memory). I figured
 out that if I could avoid acquiring `pending_builders_lock` at all then
 the build would start much faster.

--
Ticket URL: <http://trac.buildbot.net/ticket/2895#comment:9>
Buildbot <http://buildbot.net/>
Buildbot: build/test automation