Re: RFC: CONFIG_PAGE_SHIFT (aka software PAGE_SIZE)

[Dave Kleikamp <shaggy@linux.vnet.ibm.com>]

On Fri, 2007-07-13 at 17:13 +1000, David Chinner wrote:
> On Thu, Jul 12, 2007 at 09:34:57AM -0700, Dave Hansen wrote:
> > On Thu, 2007-07-12 at 18:31 +0200, Andrea Arcangeli wrote:
> > > On Fri, Jul 13, 2007 at 12:44:49AM +1000, David Chinner wrote:
> > > > That's crap. Just because a machine has lots of memory does not
> > > > make it OK to waste lots of memory.
> > > 
> > > It's not just wasted, it lowers overhead all over the place. Yes, the
> > > benefit of wasting less pagecache may largely outweight the benefit of
> > > having a larger page size, but if you've a lot of memory perhaps your
> > > working set already fits in the cache, or perhaps you don't fit in the
> > > cache regardless of the page size.
> > 
> > Have you guys seen Shaggy's page cache tails?
> > 
> > http://kernel.org/pub/linux/kernel/people/shaggy/OLS-2006/kleikamp.pdf
> > 
> > We've had the same memory waste issue on ppc64 with 64k hardware
> > pages.  
> 
> Sure. Fundamentally, though, I think it is the wrong approach to
> take - it's a workaround for a big negative side effect of
> increasing page size. It introduces lots of complexity and
> difficult-to-test corner cases; judging by the tail packing problems
> reiser3 has had over the years, it has the potential to be a
> never-ending source of data corruption bugs.

Yeah, I'm not real happy right now with the complexity of my patches.  I
had some hope that Christoph's variable page cache cleanups would
simplify some of it, but that doesn't really help.  I'm working on it
though.

> I think that fine granularity and aggregation for efficiency of
> scale is a better model to use than increasing the base page size.
> With PPC, you can handle different page sizes in the hardware (like
> MIPS) and the use of 64k base page size is an obvious workaround to
> the problem of not being able to use multiple page sizes within the
> OS.
> 
> Adding a workaround (tail packing) to address the negative side
> effects of another workaround (64k base page size) ignores the basic
> problem that has led to both these things being done: Linux does not
> support multiple page sizes natively.....

I'd much prefer having support for multiple page sizes.  I have to admit
that I don't know the VM well enough to weigh in on that debate.

Thanks,
Shaggy
-- 
David Kleikamp
IBM Linux Technology Center

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