Re: [patch] #ifdef very expensive debug check in page fault path

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

On Thu, 2008-01-17 at 00:45 +0100, Nick Piggin wrote:
> On Wed, Jan 16, 2008 at 07:01:28PM +0100, Carsten Otte wrote:
> > This patch puts #ifdef CONFIG_DEBUG_VM around a check in vm_normal_page
> > that verifies that a pfn is valid. This patch increases performance of
> > the page fault microbenchmark in lmbench by 13% and overall dbench
> > performance by 7% on s390x.  pfn_valid() is an expensive operation on
> > s390 that needs a high double digit amount of CPU cycles.
> > Nick Piggin suggested that pfn_valid() involves an array lookup on
> > systems with sparsemem, and therefore is an expensive operation there
> > too.
> > The check looks like a clear debug thing to me, it should never trigger
> > on regular kernels. And if a pte is created for an invalid pfn, we'll
> > find out once the memory gets accessed later on anyway. Please consider
> > inclusion of this patch into mm.
> > 
> > Signed-off-by: Carsten Otte <cotte@de.ibm.com>
> 
> Wow, that's a big performance hit for a few instructions ;)
> I haven't seen it to be quite so expensive on x86, but it definitely is
> not zero cost, especially with NUMA kernels. Thanks for getting those
> numbers.

These number have been a surprise. We knew that the LRA instruction we
use in pfn_valid has a cost, but from the cycle count we did not expect
that the difference in the minor fault benchmark would be 13%. Most
probably a cache effect.

I shortly discussed with Carsten what we should do with pfn_valid. One
idea was to make it a nop - always return 1. The current implementation
of pfn_valid uses the kernel address space mapping to decide if a page
frame is valid. All available memory areas that fit into the 4TB kernel
address space get mapped. If a page is mapped pfn_valid returns true.
But what is the background of pfn_valid, what does it protect against?
What is the exact semantics if pfn_valid returns true? From the name
page-frame-number-valid you could argue that it should always return
true if the number is smaller than 2**52. The number is valid, if there
is accessible memory is another question.

-- 
blue skies,
  Martin.

"Reality continues to ruin my life." - Calvin.


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