Why is machine loading rounded?

[expipiplus1]

https://github.com/NixOS/hydra/blob/master/src/hydra-queue-runner/dispatcher.cc#L131-L132

In the comparison for machine selection:

float ta = std::round(a.currentJobs / a.machine->speedFactor);
float tb = std::round(b.currentJobs / b.machine->speedFactor);

This doesn't seem necessary, and leads to incorrect results when the machines have a high speedFactor.

[expipiplus1]

@domenkozar might have figured it out:

14:48 <domenkozar> jophish: yes, one side effect of the roundf is that same machine will be reused until speedfactor is reached
14:48 <domenkozar> so it might have a consequence of less S3 downloads

[edolstra]

Yes, IIRC that was the reason.

(Looks like the C++ rewrite of build-remote got rid of the rounding BTW, which may be by accident.)

[expipiplus1]

Would it be more appropriate to divide with max jobs?

I've patched our local hydra to be a bit fairer in case anyone's interested :)

expipiplus1@73e835b

[someplaceguy]

I have simply removed the std::round on my local Hydra and it has been working great for me for a couple of months now. The workload is distributed much better now.

That said, my build machines don't do many S3 downloads as I compile everything from source, so I don't have to worry about that.

[Ericson2314]

It is back to being floats!

[Ericson2314]

Would it be more appropriate to divide with max jobs?

I've patched our local hydra to be a bit fairer in case anyone's interested :)

expipiplus1@73e835b

This is interesting and perhaps should be pursued anyways.

[someplaceguy]

It is back to being floats!

I don't understand. Wasn't this issue about the rounding, which causes the load to be distributed unevenly across the remote machines?

Based on the current code:

hydra/src/hydra-queue-runner/dispatcher.cc

Lines 234 to 235 in b3e0d9a

	float ta = std::round(a.currentJobs / a.machine->speedFactor);
	float tb = std::round(b.currentJobs / b.machine->speedFactor);

... isn't rounding still being performed, or what am I missing?