Re: [PATCH v7 2/5] arm: arm64: page_alloc: reduce unnecessary binary search in memblock_next_valid_pfn()

[Russell King - ARM Linux]

On Thu, Apr 05, 2018 at 05:50:54AM -0700, Matthew Wilcox wrote:
> On Thu, Apr 05, 2018 at 08:44:12PM +0800, Jia He wrote:
> > 
> > 
> > On 4/5/2018 7:34 PM, Matthew Wilcox Wrote:
> > > On Thu, Apr 05, 2018 at 01:04:35AM -0700, Jia He wrote:
> > > > Commit b92df1de5d28 ("mm: page_alloc: skip over regions of invalid pfns
> > > > where possible") optimized the loop in memmap_init_zone(). But there is
> > > > still some room for improvement. E.g. if pfn and pfn+1 are in the same
> > > > memblock region, we can simply pfn++ instead of doing the binary search
> > > > in memblock_next_valid_pfn.
> > > Sure, but I bet if we are >end_pfn, we're almost certainly going to the
> > > start_pfn of the next block, so why not test that as well?
> > > 
> > > > +	/* fast path, return pfn+1 if next pfn is in the same region */
> > > > +	if (early_region_idx != -1) {
> > > > +		start_pfn = PFN_DOWN(regions[early_region_idx].base);
> > > > +		end_pfn = PFN_DOWN(regions[early_region_idx].base +
> > > > +				regions[early_region_idx].size);
> > > > +
> > > > +		if (pfn >= start_pfn && pfn < end_pfn)
> > > > +			return pfn;
> > > 		early_region_idx++;
> > > 		start_pfn = PFN_DOWN(regions[early_region_idx].base);
> > > 		if (pfn >= end_pfn && pfn <= start_pfn)
> > > 			return start_pfn;
> > Thanks, thus the binary search in next step can be discarded?
> 
> I don't know all the circumstances in which this is called.  Maybe a linear
> search with memo is more appropriate than a binary search.

That's been brought up before, and the reasoning appears to be
something along the lines of...

Academics and published wisdom is that on cached architectures, binary
searches are bad because it doesn't operate efficiently due to the
overhead from having to load cache lines.  Consequently, there seems
to be a knee-jerk reaction that "all binary searches are bad, we must
eliminate them."

What is failed to be grasped here, though, is that it is typical that
the number of entries in this array tend to be small, so the entire
array takes up one or two cache lines, maybe a maximum of four lines
depending on your cache line length and number of entries.

This means that the binary search expense is reduced, and is lower
than a linear search for the majority of cases.

What is key here as far as performance is concerned is whether the
general usage of pfn_valid() by the kernel is optimal.  We should
not optimise only for the boot case, which means evaluating the
effect of these changes with _real_ workloads, not just "does my
machine boot a milliseconds faster".

-- 
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