Re: [00/17] Large Blocksize Support V3

[Andrew Morton]

On Thu, 26 Apr 2007 22:49:53 -0700 (PDT) Christoph Lameter <clameter@xxxxxxx> wrote:

> On Thu, 26 Apr 2007, Andrew Morton wrote:
> 
> > > Or make sure that truncate
> > > doesn't race on a partial *block* truncate?
> > 
> > lock four pages
> 
> You would only lock a single higher order block. Truncate works on that 
> level.

We all know that.

> If you have 4 separate pages then you need to take separate locks and you 
> may not have contiguous memory which makes the filesystem run through all 
> sorts of hoops.

This is completely incorrect.

Of *course* they're contiguous.  That's the whole point.

It's not exactly hard to lock four pages which are contiguous in pagecache,
contiguous in physical memory and are contiguous in the radix-tree.


> > I'm not saying it's especially simple, nor fast.  But it has the advantage
> > that we're not forced to use larger pages with _it's_ attendant performance
> > problems.
> 
> The patch is not about forcing to use large pages but about the option to 
> use larger pages. Its a new flexibility.

That's just spin.

> > And it doesn't introduce a rather nasty hack of pretending (in some places)
> > that pages are larger than they really are.
> 
> They are really larger. One page struct controls it all.

No it doesn't and please stop spinning.  x86 ptes map 4k pages and the core
MM needs changes to continue to work with this hack in place.

If x86 had larger pagesize we wouldn't be seeing any of this.  It is a workaround
for present-generation hardware.

> > And it has the very significant advantage that it doesn't introduce brand
> > new concepts and some complexity into core MM.
> 
> The patchset would reduce complexity and making it easy to handle the page 
> cache. Gets rid of the hacks to support larger ones right now. Its 
> straightforward, no new locking, very much a cleanup patch.

Were any cleanups made which were not also applicable as standalone things
to mainline?

> > And make no mistake: the latter disadvantage is huge.  Because if we do the
> > PAGE_CACHE_SIZE hack (sorry, but it _is_), we have to do it *for ever*. 
> > Maintaining and enhancing core MM and VFS becomes harder and more costly
> > and slower and more buggy *for ever*.  The ramp for people to become
> > competent on core MM becomes longer.  Our developer pool becomes smaller, and
> > proportionally less skilled.
> 
> No it becomes easier. Look at the patchset. It cleans up a huge mess.

I see no cleanups which are not also applicable to mainline.

> What is hacky about it?

It pretends that pages are large than they actually are, forcing the
pte-management code to also play along with the pretence.

Pages *aren't* 16k.  They're 4k.

> It is consistently using larger pages for the page 
> cache and it integrates nicely into the VM.


> > And hardware gets better.  If Intel & AMD come out with a 16k pagesize
> > option in a couple of years we'll look pretty dumb.  If the problems which
> > you're presently having with that controller get sorted out in the next
> > generation of the hardware, we'll also look pretty dumb.
> 
> We are currently looking dumb and unable to deal with the hardware. Yes 
> we can pressure the hardware vendors to produce hardware conforming to our 
> specifications but I always thought that was how another company operates.

That's spin as well.

Please address my point: if in five years time x86 has larger or varible
pagesize, this code will be a permanent millstone around our necks which we
*should not have merged*.

And if in five years time x86 does not have larger pagesize support then
the manufacturers would have decided that 4k pages are not a performance
problem, so we again should not have merged this code.

> > As always, there are tradeoffs.  We can see the cons, and they are very
> > significant.  We don't yet know the pros.  Perhaps they will be similarly
> > significant.  But I don't believe that the larger PAGE_CACHE_SIZE hack
> > (sorry) is the only way in which they can be realised.
> 
> It is the most consistent solution that avoid the proliferation of further 
> hacks to address the large blocksize.

You cannot say this.  I'm sitting here *watching* you refuse to seriously
consider alternatives.

And you've conspicuously failed to address my point regarding the
*permanent* additional maintenance cost.


Anyway.  Let's await those performance numbers.  If they're notably good,
and if we judge that this goodness will be realised on more than one
arguably-crippled present-day disk adapter then we can evaluate the
*various* options which we have for stuffing more data into that adapter.


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