Re: [PATCH 10/10] alternate 4-level page tables patches

[Nick Piggin <nickpiggin@yahoo.com.au>]

Andi Kleen wrote:
> On Sat, Dec 18, 2004 at 06:01:05PM +1100, Nick Piggin wrote:
> 
>>10/10
> 
> 
>>
>>Convert some pagetable walking functions over to be inline where
>>they are only used once. This is worth a percent or so on lmbench
>>fork.
> 
> 
> Any modern gcc (3.4+ or 3.3-hammer) should use unit-at-a-time anyways,
> which automatically inlines all static functions that are only used once.
> 
> I like it because during debugging you can turn it off and it makes
> it much easier to read oopses when not everything is inlined.  And 
> when turned on it generates much smaller and faster as you've shown
> code.
> 

Yep, that makes a lot of sense.

> Ok except on i386 where someone decided to explicitely turn it off 
> all the time :/
> 
> I've been reenabling it on the suse kernel for a long time because
> it doesn't seem to have any bad side effects and makes the code
> considerably smaller.  It would be better to just turn it on in mainline 
> again, then you'll see much more gain everywhere.
> 

I won't get into this argument ;)

But I'll just say that this inline patch isn't very important at all -
it seems to be worth about 1% at best.

> BTW we can do much better with all the page table walking by
> adding some bitmaps about used ptes to struct page and skipping
> holes quickly. DaveM has a patch for that in the queue, I hope a patch 
> similar to his can be added once 4level page tables are in.
> 

Hmm, haven't seen them. Would be interesting - I guess you can get
a pretty big cache saving by testing a single bit rather than a
full word, assuming the common case is pretty sparse. I wonder how
it goes in practice?
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