Re: [gentoo-user] emerge -j, make -j and make -l

[Pandu Poluan <pandu@poluan.info>]

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On Nov 27, 2011 5:12 PM, "Michael Mol" <mikemol@gmail.com> wrote:
>
> I figure that the optimal number of simultaneous CPU-consuming
> processes is going to be the number of CPU cores, plus enough to keep
> the CPU occupied while others are blocked on I/O. That's the same
> reasoning that drives the selection of a -j number, really.
>
> If I read make's man page correctly, -l acts as a threshold, choosing
> not to spawn an additional child process if the system load average is
> above a certain value Since system load is a count of actively running
> and ready-to-run processes, you want it to be very close to your
> number of logical cores[1].
>
> Since it's going to be a spot decision for Make as to whether or not
> to spawn another child (if it hits its limit, it's not going to check
> again until after one of its children returns), there will be many
> race cases where the load average is high when it looks, but some
> other processes will return shortly afterward.[2] That means adding a
> process or two for a fudge factor.
>
> That's a lot of guess, though, and it still comes down to guess-and-check.
>
> emerge -j8 @world # MAKEOPTS="-j16 -l10"
>
> Was the first combination I tried. This completed in 89 minutes.
>
> emerge -j8 @world # MAKEOPT="-j16 -l8"
>
> Was the second. This took significantly longer.
>
> I haven't tried higher than -l10; I needed this box to do be able to
> do things, which meant installing more software. I've gone from 177
> packages to 466.
>
> [1] I don't have a hyperthreading system available, but I suspect that
> this is also going to be true of logical cores; It's my understanding
> that the overhead from overcommitting CPU comes primarily from context
> switching between processors, and hyperthreading adds CPU hardware
> specifically to reduce the need to context-switch in splitting
> physical CPU resources between threads/processes. So while you'd lose
> a little speed for an individual thread, you would gain it back in
> aggregate over both threads.
>
> [2] There would also be cases where the load average is low, such as
> if a Make recipe calls for a significant bit of I/O before it consumes
> a great deal of CPU, but a simple 7200rpm SATA disk appears to be
> sufficiently fast that this case is less frequent.

Here's my experience:

I always experience emerge failures on my Gentoo VMs if I use
MAKEOPTS=-j>3. Not all packages, but many. Including, IIRC, glibc and gcc.

This happens even if I make sure that there's just one emerge job being
done. And this happens even if I allocate more vCPUs than -j, on VMware and
XenServer alike.

I don't know where the 'blame' lies, but I've found myself standardizing on
MAKEOPTS=-j3, and PORTAGE_DEFAULT_OPTS="--jobs
--load-average=<1.6*num_of_vCPU>"

(Yes, no explicit number of jobs. The newer portages are smart enough to
keep starting new jobs until the load number is reached)

Rgds,

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