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

[Mike Yoknis <mike.yoknis@hp.com>]

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.
 
Mike Yoknis