Re: [RFC PATCH 0/5] Support multiple pages allocation

[Joonsoo Kim]

On Thu, Jul 04, 2013 at 12:01:43AM +0800, Zhang Yanfei wrote:
> On 07/03/2013 11:51 PM, Zhang Yanfei wrote:
> > On 07/03/2013 11:28 PM, Michal Hocko wrote:
> >> On Wed 03-07-13 17:34:15, Joonsoo Kim wrote:
> >> [...]
> >>> For one page allocation at once, this patchset makes allocator slower than
> >>> before (-5%). 
> >>
> >> Slowing down the most used path is a no-go. Where does this slow down
> >> come from?
> > 
> > I guess, it might be: for one page allocation at once, comparing to the original
> > code, this patch adds two parameters nr_pages and pages and will do extra checks
> > for the parameter nr_pages in the allocation path.
> > 
> 
> If so, adding a separate path for the multiple allocations seems better.

Hello, all.

I modify the code for optimizing one page allocation via likely macro.
I attach a new one at the end of this mail.

In this case, performance degradation for one page allocation at once is -2.5%.
I guess, remained overhead comes from two added parameters.
Is it unreasonable cost to support this new feature?
I think that readahead path is one of the most used path, so this penalty looks
endurable. And after supporting this feature, we can find more use cases.

I will try to add a new function for the multiple allocations and test it. But,
IMHO, adding a new function is not good idea, because we should duplicate
various checks which are already in __alloc_pages_nodemask and even if
we introduce a new function, we cannot avoid to pass two parameters
to get_page_from_freelist(), so slight performance degradation on
one page allocation is inevitable. Anyway, I will do and test it.

Thanks.

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