Re: [patch 3/3] slub: sort parital list when thrashing

[David Rientjes]

On Mon, 30 Mar 2009, Christoph Lameter wrote:

> > @@ -1720,6 +1727,15 @@ checks_ok:
> >  	if (unlikely(!prior)) {
> >  		add_partial(get_node(s, page_to_nid(page)), page, 1);
> >  		stat(c, FREE_ADD_PARTIAL);
> > +	} else if (c->slowpath_allocs >= SLAB_THRASHING_THRESHOLD) {
> > +		/*
> > +		 * If the cache is actively slab thrashing, it's necessary to
> > +		 * move partial slabs to the head of the list so there isn't
> > +		 * excessive partial list scanning while holding list_lock.
> > +		 */
> > +		if (!skip_partial(s, page))
> > +			move_partial_to_head(get_node(s, page_to_nid(page)),
> > +					     page);
> >  	}
> >
> >  out_unlock:
> >
> 
> This again adds code to a pretty hot path.
> 
> What is the impact of the additional hot path code?
> 

I'll be collecting more data now that there's a general desire for a 
default slab_thrash_ratio value, so we'll implicitly see the performance 
degradation for non-thrashing slab caches.  Mel had suggested a couple of 
benchmarks to try and my hypothesis is that they won't regress with a 
default ratio of 20 for all caches with >= 20 objects per slab (at least a 
4 object threshold for determining thrash).
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