Re: Get rid of scan_control

[Christoph Lameter <clameter@engr.sgi.com>]

On Sun, 12 Feb 2006, Nick Piggin wrote:

> > Could we at least pass the number of pages reclaimed back as the return
> > value of the functions? I believe most of the savings that Andrew saw was
> > due to the number of reclaimed pages being processed directly in registers.
> 
> What savings are you interested in, exactly? Your initial patch
> would definitely have slowed down page reclaim on big systems
> due to the read_page_state...

The patch that put the whole calculation into a separate block that 
is only executed for the swap case would have taken care of that.


> I think most of the cost apart from locking (because that will
> depend on contention) is hitting random cachelines of struct pages
> then hitting random radix tree cachelines to remove them. Not
> much you can do about that.
> 
> That said I'm never against microoptimisations provided they
> weigh in on the right side of the (subjective) complexity /
> improvement ratio.

Its a bit strange if you call a function and then access a structure 
member to get the result. Locating parameter in a structure makes it
impossible to see what is passed to a function when it is 
called.

It is also something that will make it difficult for compilers to do
a good job. Flow control is easier to optimize for a local variable
than for a pointer into a struct that may have been modified elsewhere.

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