Re: visualize build parallelism, duration of build steps?

From some experiments MongoDB ran, you can saturate the scheduler without
maxing out available CPU's with the current taskmaster.
They produced a PR with some patches to improve the job finding and
dispatch logic.

If you're hitting a wall where you CPU cores are idle, that PR's patch
might be interesting.
Though it could be that SCons schedules lots of object compiles -> fewer
ar/shared library linking -> fewer programs could also be leading to under
utilized cores later in your build and/or as part of an incremental build.

Hope that helps,
Bill
SCons Project Co-Manager

On Tue, Oct 19, 2021 at 3:55 PM Gabe Black <[email protected]> wrote:

> There definitely seems to be a build size component to this. I have a
> series of changes which don't change how anything is built, but tell SCons
> about everything and then select what to build with different dependencies
> instead of using python logic to selectively set up build rules. With -j
> set to its previous saturation point performance wise, this actually
> increased build time by about 7-8%. I deleted the build and reran to make
> sure there weren't caching effects impacting the results, and they stayed
> consistent.
>
> Gabe
>
> On Tue, Oct 19, 2021 at 6:59 AM Mats Wichmann <[email protected]> wrote:
>
>> On 10/18/21 16:45, Gabe Black 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?
>>
>> The taskmastertrace is generally the way to extract this information.
>> It's a bit like trying to drink from a firehose, though, so a tool to
>> process it would likely help. Don't know if there are any of those
>> floating around.
>>
>>
>> > I've had a suspicion for a few years that there might be a task
>> scheduling bug where at certain points a super task seems to come along and
>> stop any other tasks from being scheduled until it is done.
>> There's been some discussion of this over time.  It appears the way
>> scheduling is done may be prone to getting blocked by a long-running
>> jobs.  One tuning of the algorithm was proposed in a PR which hasn't
>> received much action - opinions about whether this had really pinpointed
>> an underlying issue were not unanimous.
>>
>> https://github.com/SCons/scons/pull/3386
>>
>> There's also been talk about maybe not having all the members of the
>> pool of job threads be handled identically, so there's a place you could
>> put a long-running job to get it started as soon as possible, and not
>> wait behind a bunch of shorter ones, which could then compete for the
>> other threads and maybe get closer to finishing at the same times.
>> Making decisions based on length of jobs would require some sort of
>> profiling capability so the scheduler has the information available.
>>
>> On the whole I think it's been posited that there are performance issues
>> when the build gets very big, and not sure it's well understood why.
>> The taskmaster is a pretty complex piece of code and it was heavily
>> tuned "in the old days" (~15 years ago).  Be nice to have better ways to
>> characterize what happens on modern hardware configurations with
>> potentially larger build projects than when that work was done.
>>
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