Re: Page allocator: Single Zone optimizations

[Andi Kleen <ak@suse.de>]

On Monday 06 November 2006 18:00, Christoph Lameter wrote:
> On Mon, 6 Nov 2006, Andi Kleen wrote:
> 
> > > Because acceses to the structure can occur after kfree. The RCU 
> > > implementation only delays the destruction of the slab. Locks are always 
> > > in a definite state regardless if the object is in use or not.
> > 
> > Only objects that have been used at least once can be still visible. And 
> > those would be still constructed of course -- just after the kmem_cache_alloc,
> > not inside. For those that have never been used it shouldn't matter.
> 
> Constructors are only called on allocation of the slab, not on 
> kmem_cache_alloc. 

I know this.

> And you are right: It does not matter for those that  
> have never been used.

This means it is fine to replace the constructor with an function
that runs after kmem_cache_alloc() in this case.

> > > I think this is an attempt to avoid having to initialize pmds/pgds after 
> > > intializaiton and also the use of the slab caches keeps the cache lines 
> > > hot.
> > 
> > Ah, we got __GFP_ZERO for that, although it never quite did the work 
> > completely. I'm not sure it helps a lot anyways
> 
> Not exactly. The implementation in the i386 arch code avoids the 
> __GFP_ZERO by relying on empty pgd/pmds be zero. But you could copy Robin 
> Holt's implementation via page lists from ia64 that does the saem. It
> avoids the constructor/destructors and slab use. It is cleaner and 
> probably faster.

i386 used to have such lists some time ago too, until they were removed.

What I meant: some time ago i had patches to add a __GFP_ZERO queue to the
page allocator. The page allocator would handle all this for everybody. 
For various reasons they never got pushed.

But I am not sure it is worth it all that much because there are not
that many PMDs allocated typically.

-Andi

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