Re: [PATCH 1/3] slub: set a criteria for slub node partial adding

[Shaohua Li]

On Wed, 2011-12-07 at 15:28 +0800, David Rientjes wrote:
> On Wed, 7 Dec 2011, Shaohua Li wrote:
> 
> > interesting. I did similar experiment before (try to sort the page
> > according to free number), but it appears quite hard. The free number of
> > a page is dynamic, eg more slabs can be freed when the page is in
> > partial list. And in netperf test, the partial list could be very very
> > long. Can you post your patch, I definitely what to look at it.
> 
> It was over a couple of years ago and the slub code has changed 
> significantly since then, but you can see the general concept of the "slab 
> thrashing" problem with netperf and my solution back then:
> 
> 	http://marc.info/?l=linux-kernel&m=123839191416478
> 	http://marc.info/?l=linux-kernel&m=123839203016592
> 	http://marc.info/?l=linux-kernel&m=123839202916583
> 
> I also had a separate patchset that, instead of this approach, would just 
> iterate through the partial list in get_partial_node() looking for 
> anything where the number of free objects met a certain threshold, which 
> still defaulted to 25% and instantly picked it.  The overhead was taking 
> slab_lock() for each page, but that was nullified by the performance 
> speedup of using the alloc fastpath a majority of the time for both 
> kmalloc-256 and kmalloc-2k when in the past it had only been able to serve 
> one or two allocs.  If no partial slab met the threshold, the slab_lock() 
> is held of the partial slab with the most free objects and returned 
> instead.
With the per-cpu partial list, I didn't see any workload which is still
suffering from the list lock, so I suppose both the trashing approach
and pick 25% used slab approach don't help. The per-cpu partial list
flushes the whole per-cpu partial list after s->cpu_partial objects are
freed, this is a little aggressive, because the per-cpu partial list
need refilled again soon after an allocation. I had experiment to have
separate per-cpu alloc/free partial list, which can avoid this. but
again, I didn't see any workload still suffering list lock issue even
with netperf which stress slub much. did you see such workload?

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