[Scons-users] visualize build parallelism, duration of build steps?

[Stijn De Ruyck]

FWIW, during my own testing, I noticed a performance benefit for GCC compilations up to the number of physical cores +2. So for a hyperthreaded 12 core system with enough memory available, I would see gains up to -j14. The other 10 virtual cores wouldn’t help much. Anything higher than -j14 would not result in decreased compilation time, only increased memory consumption. You also need to take into account memory & disk bandwidth bottlenecks when using high parallelism on only a single system.

I’m not aware of turn-keyish integration for distributed builds, but we have used DistCC with ease in the past, there’s not much setup involved, and now use Scons with LSF instead.

Stijn

From: Scons-users <scons-users-bounces at scons.org> On Behalf Of Gabe Black
Sent: Tuesday, October 19, 2021 11:56
To: SCons users mailing list <scons-users at scons.org>
Subject: Re: [Scons-users] visualize build parallelism, duration of build steps?

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To follow up with some measurements, I found that my build performance scales logarithmically with -j, up to the number of threads in my system. I have a 12 core CPU with 2 threads per core, so that is up to 24 threads. After that point, performance stops improving and actually degrades slightly. By logarithmically, I mean that the graph of the log of the -j value tracks the decrease in run time of a clean build reasonably well. Or in other words, I would expect going from -j4 to -j16 to improve build time by a factor of 2x, and not a factor of 4x.

Is this expected? Is there a way to diagnose what keeping our build from scaling roughly linearly? We have many individual object files which should be buildable in parallel, so I'm not sure what the bottleneck is. There curve seems to be fairly smooth and not a hard drop off, so I don't think we're hitting a specific limit in memory bandwidth or capacity, overflowing the disk cache, etc.

Gabe

On Mon, Oct 18, 2021 at 3:45 PM Gabe Black <gabe.black at gmail.com<mailto:gabe.black at gmail.com>> wrote:
Hi folks, sorry if this is a really obvious question, but is there a command line flag or tool or something to visualize how parallel a SCons build is, if there are any bottlenecks, if there are abnormally long running build steps, etc?

I don't have an immediate pressing need for a tool like that, but I think it might be helpful identifying places our build could be tightened up. It could and also help get a rough idea of how well the build would scale to a very parallel build machine, ie. what might build time be with 32 cores? 64 cores? 128 cores? etc. How much memory would you need for 128 cores to avoid memory bottlenecks?

Somewhat on that topic, are there any relatively easy to set up distributed build mechanisms for SCons? I'm sure you could plug distcc into it for instance, but is there anything which has some nice, turn key(ish?) integration?

Thanks!
Gabe
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