Re: [PATCH] mm: memmap_init_zone() performance improvement

[Mel Gorman <mgorman@suse.de>]

On Fri, Oct 19, 2012 at 01:53:18PM -0600, Mike Yoknis wrote:
> On Tue, 2012-10-09 at 08:56 -0600, Mike Yoknis wrote:
> > On Mon, 2012-10-08 at 16:16 +0100, Mel Gorman wrote:
> > > On Wed, Oct 03, 2012 at 08:56:14AM -0600, Mike Yoknis wrote:
> > > > memmap_init_zone() loops through every Page Frame Number (pfn),
> > > > including pfn values that are within the gaps between existing
> > > > memory sections.  The unneeded looping will become a boot
> > > > performance issue when machines configure larger memory ranges
> > > > that will contain larger and more numerous gaps.
> > > > 
> > > > The code will skip across invalid sections to reduce the
> > > > number of loops executed.
> > > > 
> > > > Signed-off-by: Mike Yoknis <mike.yoknis@hp.com>
> > > 
> > > I do not see the need for
> > > the additional complexity unless you can show it makes a big difference
> > > to boot times.
> > > 
> > 
> > Mel,
> > 
> > Let me pass along the numbers I have.  We have what we call an
> > "architectural simulator".  It is a computer program that pretends that
> > it is a computer system.  We use it to test the firmware before real
> > hardware is available.  We have booted Linux on our simulator.  As you
> > would expect it takes longer to boot on the simulator than it does on
> > real hardware.
> > 
> > With my patch - boot time 41 minutes
> > Without patch - boot time 94 minutes
> > 
> > These numbers do not scale linearly to real hardware.  But indicate to
> > me a place where Linux can be improved.
> > 
> > Mike Yoknis
> > 
> Mel,
> I finally got access to prototype hardware.  
> It is a relatively small machine with only 64GB of RAM.
>  
> I put in a time measurement by reading the TSC register.
> I booted both with and without my patch -
>  
> Without patch -
> [    0.000000]   Normal zone: 13400064 pages, LIFO batch:31
> [    0.000000] memmap_init_zone() enter 1404184834218
> [    0.000000] memmap_init_zone() exit  1411174884438  diff = 6990050220
>  
> With patch -
> [    0.000000]   Normal zone: 13400064 pages, LIFO batch:31
> [    0.000000] memmap_init_zone() enter 1555530050778
> [    0.000000] memmap_init_zone() exit  1559379204643  diff = 3849153865
>  
> This shows that without the patch the routine spends 45% 
> of its time spinning unnecessarily.
>  

I'm travelling at the moment so apologies that I have not followed up on
this. My problem is still the same with the patch - it changes more
headers than is necessary and it is sparsemem specific. At minimum, try
the suggestion of 

if (!early_pfn_valid(pfn)) {
      pfn = ALIGN(pfn + MAX_ORDER_NR_PAGES, MAX_ORDER_NR_PAGES) - 1;
      continue;
}

and see how much it gains you as it should work on all memory models. If
it turns out that you really need to skip whole sections then the strice
could MAX_ORDER_NR_PAGES on all memory models except sparsemem where the
stride would be PAGES_PER_SECTION

-- 
Mel Gorman
SUSE Labs

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