Re: [PATCH 04/33] autonuma: define _PAGE_NUMA

[Andrea Arcangeli <aarcange@redhat.com>]

On Thu, Oct 11, 2012 at 12:01:37PM +0100, Mel Gorman wrote:
> On Thu, Oct 04, 2012 at 01:50:46AM +0200, Andrea Arcangeli wrote:
> > The objective of _PAGE_NUMA is to be able to trigger NUMA hinting page
> > faults to identify the per NUMA node working set of the thread at
> > runtime.
> > 
> > Arming the NUMA hinting page fault mechanism works similarly to
> > setting up a mprotect(PROT_NONE) virtual range: the present bit is
> > cleared at the same time that _PAGE_NUMA is set, so when the fault
> > triggers we can identify it as a NUMA hinting page fault.
> > 
> 
> That implies that there is an atomic update requirement or at least
> an ordering requirement -- present bit must be cleared before setting
> NUMA bit. No doubt it'll be clear later in the series how this is
> accomplished. What you propose seems ok but it all depends how it's
> implemented so I'm leaving my ack off this particular patch for now.

Correct. The switch is done atomically (clear _PAGE_PRESENT at the
same time _PAGE_NUMA is set). The tlb flush is deferred (it's batched
to avoid firing an IPI for every pte/pmd_numa we establish).

It's still similar to setting a range PROT_NONE (except the way
_PAGE_PROTNONE and _PAGE_NUMA works is the opposite, and they are
mutually exclusive, so they can easily share the same pte/pmd
bitflag). Except PROT_NONE must be synchronous, _PAGE_NUMA is set lazily.

The NUMA hinting page fault also won't require any TLB flush ever.

So the whole process (establish/teardown) has an incredibly low TLB
flushing cost.

The only fixed cost is in knuma_scand and the enter/exit kernel for
every not-shared page every 10 sec (or whatever you set the duration
of a knuma_scand pass in sysfs).

Furthermore, if the pmd_scan mode is activated, I guarantee there's at
max 1 NUMA hinting page fault every 2m virtual region (even if some
accuracy is lost). You can try to set scan_pmd = 0 in sysfs and also
to disable THP (echo never >enabled) to measure the exact cost per 4k
page. It's hardly measurable here. With THP the fault is also 1 every
2m virtual region but no accuracy is lost in that case (or more
precisely, there's no way to get more accuracy than that as we deal
with a pmd).