On Sat, 3 Nov 2007, Christoph Lameter wrote:
> On Sat, 3 Nov 2007, Hugh Dickins wrote:
>
> > I was afraid you might say something like that.
> > Perhaps it'll be a patch I need to use in my own builds.
> > Though I'd have thought others would want that accuracy too.
> > Didn't SLAB give it? (The "r*gr*ss**n" word!)
>
> Slab also only counts objects that are not in the queues. See free_block()
> f.e.
I'll take your word for it, and apologize for my slur on slub!
(Slub has a great deal to admire in it, I should say.)
>
> We could improve the situation by flushing all cpu slabs before counts are
> determined.
>
> Which can be done manually. Run
>
> slabinfo -s
>
> and then look at the numbers.
Mmm, I'd been doing slabinfo -v sometimes. These are fine in some
situations, but it's always better when the observer can avoid
interfering with the observed. Impossible, we know, but...
Also, many caches too quickly re-equip themselves
with cpu slabs which again obscure the numbers.
> > > Adds to much overhead to the fast paths
> >
> > You've come to that conclusion very quickly!
>
> I have just spend a few weeks optimizing the fast and slow paths and there
> is some additional overhead that I am still trying to eliminate.
>
> > Any numbers to back it up?
>
> The performance in the fast paths depends on updating only a single word
> for an allocation. Adding another counter makes that impossible.
Gosh, that's a tighter corner than any I've been in.
>
> See the recent post on SLUB regression on SMP.
I'll have to read up on that, thanks for the pointer.
Hugh
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