Re: [patch 6/8] slub: Adjust order boundaries and minimum objects per slab.

[Matt Mackall <mpm@selenic.com>]

On Mon, Mar 03, 2008 at 09:52:55AM -0800, Christoph Lameter wrote:
> On Sat, 1 Mar 2008, Pekka J Enberg wrote:
> 
> > On Fri, 29 Feb 2008, Christoph Lameter wrote:
> > > The defaults for slab are also 60 objects per slab. The PAGE_SHIFT says 
> > > nothing about the big iron. Our new big irons have a page shift of 12 and 
> > > are x86_64.
> > 
> > Where is that objects per slab limit? I only see calculate_slab_order() 
> > trying out bunch of page orders until we hit "acceptable" internal 
> > fragmentation. Also keep in mind how badly SLAB compares to SLUB and SLOB 
> > in terms of memory efficiency.
> 
> slub_min_objects sets that limit.
>  
> > On Fri, 29 Feb 2008, Christoph Lameter wrote:
> > > We could drop the limit if CONFIG_EMBEDDED is set but then this may waste 
> > > space. A higher order allows slub to reach a higher object density (in 
> > > particular for objects 500-2000 bytes size).
> > 
> > I am more worried about memory allocated for objects that are not used 
> > rather than memory wasted due to bad fitting.
> 
> Is there any way to quantify this? This is likely only an effect that 
> mostly matters for rarely used slabs (the merging reduces that effect). 
> F.e. fitting more inodes or dentries into a single slab increases object 
> density.

On the other hand, a single object can now pin 64k in memory rather
than 4k. So when we collapse some cache under memory pressure, we're
not likely to free as much.

I know you've put a lot of effort into dealing with the dcache and
icache instances of this, but this could very well offset most of that.

Also, we might consider only allocating an order-1 slab if we've
filled an order-0, and so on. When we hit pressure, we kick our
order counter back to 0.

-- 
Mathematics is the supreme nostalgia of our time.

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