Re: [PATCH 11/39] autonuma: CPU follow memory algorithm

[Andrea Arcangeli <aarcange@redhat.com>]

Hi,

On Mon, Mar 26, 2012 at 12:58:05PM -0700, Linus Torvalds wrote:
> On Mar 26, 2012 12:45 PM, "Andrea Arcangeli" <aarcange@redhat.com> wrote:
> >
> > As I wrote in the comment before the function, math speaking, this
> > looks like O(N) but it is O(1), not O(N) nor O(N^2). This is because N
> > = NR_CPUS = 1.
> 
> That's just stupid sophistry.

I agree, this is why I warned everyone in the comment before the
function with the adjective "misleading":

 * O(1) misleading math
 * aside, the number of cachelines touched with thousands of CPU might
 * make it measurable.

> No, you can't just say that it's limited to some large constant, and thus
> the same as O(1).

I pointed out it is O(1) just because if we use the O notation we may
as well do the math right about it.

I may not have been clear but I never meant that because it is O(1)
(NR_CPUS constant) it means it's already ok as it is now.

> 
> That's the worst kind of lie: something that's technically true if you look
> at it a certain stupid way, but isn't actually true in practice.
> 
> It's clearly O(n) in number of CPUs, and people told you it can't go into
> the scheduler. Stop arguing idiotic things. Just say you'll fix it, instead
> of looking like a tool.

About fixing it, this can be called at a regular interval like
load_balance() (which also has an higher cost than the per-cpu
schedule fast path, in having to walk over the other CPU runqueues) or
to be more integrated within CFS so it doesn't need to be called at
all.

I didn't think it was urgent to fix (also because it has a debug
benefit to keep it like this in the short term), but I definitely
intended to fix it.

I also would welcome people who knows the scheduler so much better
than me to rewrite or fix it as they like it.

To be crystal clear: I totally agree to fix this, in the comment
before the code I wrote:

 * it's good in the
 * short term for stressing the algorithm.

I probably wasn't clear enough, but I already implicitly meant it
shall be optimized further later.

If there's a slight disagreement is only on the "urgency" to fix it but
I will certainly change my priorities on this after reading your
comments!

Thanks for looking into this.
Andrea

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