package building performance (was: Re: FreeBSD on AMD Epyc boards)

[Mark Linimon]

On Wed, Feb 14, 2018 at 09:15:53AM +0100, Kurt Jaeger wrote:
> On the plus side: 16+16 cores, on the minus: A low CPU tact of 2.2 GHz.
> Would a box like this be better for a package build host instead of 4+4 cores
> with 3.x GHz ?

In my experience, "it depends".

I think that above a certain number of cores, I/O will dominate.  I _think_;
I have never done any metrics on any of this.

The dominant term of the equation is, as you might guess, RAM.  Previous
experience suggests that you need at least 2GB per build.  By default,
nbuilds is set equal to ncores.  Less than 2GB-per and you're going to be
unhappy.

(It's true that for modern systems, where large amounts of RAM are standard,
that this is probably no longer a concern.)

Put it this way: with 4 cores and 16GB and netbooting (7GB of which was
devoted to md(4)), I was having lots of problems on powerpc64.  The same
machine with 64GB gives me no problems.

My guess is that after RAM, there is I/O, ncores, and speed.  But I'm just
speculating.

mcl