Re: [rfc] increase struct page size?!

[David Miller <davem@davemloft.net>]

From: Nick Piggin <npiggin@suse.de>
Date: Fri, 18 May 2007 06:08:54 +0200

> I'd like to be the first to propose an increase to the size of struct page
> just for the sake of increasing it!
> 
> If we add 8 bytes to struct page on 64-bit machines, it becomes 64 bytes,
> which is quite a nice number for cache purposes.
> 
> However we don't have to let those 8 bytes go to waste: we can use them
> to store the virtual address of the page, which kind of makes sense for
> 64-bit, because they can likely to use complicated memory models.
> 
> I'd say all up this is going to decrease overall cache footprint in 
> fastpaths, both by reducing text and data footprint of page_address and
> related operations, and by reducing cacheline footprint of most batched
> operations on struct pages.
> 
> Flame away :)

I've toyed with this several times on sparc64, and in my experience
the extra memory reference on page->virtual costs on average about the
same as the non-power-of-2 pointer arithmetic.

The decision is absolutely arbitrary performance wise, but if you
consider the memory wastage on enormous systems going without
page->virtual I think is clearly better.
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