One build was faster than all the others?

acrichton's Avatar


12 Jan, 2018 07:32 PM

Hello! Our builds at rust-lang/rust have been slowing down for some time now (not Appveyor's fault, our own), so we've been doing some analysis of build times historically to see what's up. In doing so we ran across a curious build that happened recently. In the past few months our builds take about 2.5 hours per run (for this one configuration), but it turns out for one build our build ran an hour more quickly!

This build specifically took 1.5hrs to complete while the next build took 2.5hrs to complete. The longer build definitely didn't regress performance by over and hour and it appears that the faster build almost everything ran faster than the next build.

The best explanation we can come up with is that something about the VM's hardware changed (or something like that?). I was wondering if y'all could help us dig into this to see what happened? Did these two builds run on VMs with similar resources? Or did one run on a faster VM?

I think historically we were made aware that VMs can be slower when the whole system is under load, but 1hr faster is crazy faster! We're assuming that it doesn't have to do with load, but we're curious ourselves still!

  1. 1 Posted by acrichton on 12 Jan, 2018 07:51 PM

    acrichton's Avatar

    Oh so actually one of the steps of our build counts the number of cpus on the machine and executes that with a parameter for a command, showing that the fast build ran with 4 cores and the slow build ran with 2 cores.

    If that's the case, how come one build ran with 4 cores? It's not at all a problem for us, I'm mostly just curious now!

  2. Support Staff 2 Posted by Feodor Fitsner on 13 Jan, 2018 01:39 AM

    Feodor Fitsner's Avatar

    Wow, indeed that particular cloud running that build at that time had 4 cores configured. It's "fixed" now, thanks for the catch :)

    Btw, if we figure out the way to give specific accounts VMs with more cores (sometimes the server has spare computation power) would you be interested to build on 4 cores in exchange for more pricey subscription?

  3. 3 Posted by acrichton on 13 Jan, 2018 06:02 PM

    acrichton's Avatar

    Ok thanks for the info! That was actually my next question too! Along those lines, what would it look like for our VMs to get bumped from 2 cores to 4? (pricing wise and such)

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