On Mon, 2012-03-19 at 14:04 +0100, Andrea Arcangeli wrote:

About the cost of the actual pagetable scanner, you're not being
rational about it. You should measure it for once, take khugepaged
make it scan 1G of memory per millisecond and measure the cost.

Death by a thousand cuts.. 

You keep complaining about the unaccountability of the pagetable
scanners in terms of process load, and that's a red herring as far as
I can tell. The irqs and ksoftirqd load in a busy server, is likely
much higher than whatever happens at the pagetable scanner level (sure
thing for khugepaged and by an huge order of magnitude so). 

Who says I agree with ksoftirqd? I would love to get rid of all things
softirq. And I also think workqueues are over-/ab-used.

I don't
think this is a relevant concern anyway because the pagetable scanners
go over all memory in a equal amount so the cost would be evenly
distributed for all processes over time (the same cannot be said about
the irqs and ksoftrqid that will benefit only a few processes doing
I/O). 

So what about the case where all I do is compile kernels and we already
have near perfect locality because everything is short running? You're
still scanning that memory, and I get no benefit.

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