On Thu 21-03-13 11:55:16, Ingo Molnar wrote:
>
> * Russ Anderson <rja@xxxxxxx> wrote:
>
> > When booting on a large memory system, the kernel spends
> > considerable time in memmap_init_zone() setting up memory zones.
> > Analysis shows significant time spent in __early_pfn_to_nid().
> >
> > The routine memmap_init_zone() checks each PFN to verify the
> > nid is valid. __early_pfn_to_nid() sequentially scans the list of
> > pfn ranges to find the right range and returns the nid. This does
> > not scale well. On a 4 TB (single rack) system there are 308
> > memory ranges to scan. The higher the PFN the more time spent
> > sequentially spinning through memory ranges.
> >
> > Since memmap_init_zone() increments pfn, it will almost always be
> > looking for the same range as the previous pfn, so check that
> > range first. If it is in the same range, return that nid.
> > If not, scan the list as before.
> >
> > A 4 TB (single rack) UV1 system takes 512 seconds to get through
> > the zone code. This performance optimization reduces the time
> > by 189 seconds, a 36% improvement.
> >
> > A 2 TB (single rack) UV2 system goes from 212.7 seconds to 99.8 seconds,
> > a 112.9 second (53%) reduction.
>
> Nice speedup!
>
> A minor nit, in addition to Andrew's suggestion about wrapping
> __early_pfn_to_nid():
>
> > Index: linux/mm/page_alloc.c
> > ===================================================================
> > --- linux.orig/mm/page_alloc.c 2013-03-18 10:52:11.510988843 -0500
> > +++ linux/mm/page_alloc.c 2013-03-18 10:52:14.214931348 -0500
> > @@ -4161,10 +4161,19 @@ int __meminit __early_pfn_to_nid(unsigne
> > {
> > unsigned long start_pfn, end_pfn;
> > int i, nid;
> > + static unsigned long last_start_pfn, last_end_pfn;
> > + static int last_nid;
>
> Please move these globals out of function local scope, to make it more
> apparent that they are not on-stack. I only noticed it in the second pass.
Wouldn't this just add more confision with other _pfn variables? (e.g.
{min,max}_low_pfn and others)
IMO the local scope is more obvious as this is and should only be used
for caching purposes.
--
Michal Hocko
SUSE Labs
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>