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

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Date:	Mon, 12 Dec 2011 12:48:24 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	"Alex,Shi" <alex.shi@...el.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Christoph Lameter <cl@...ux.com>,
	"penberg@...nel.org" <penberg@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 1/3] slub: set a criteria for slub node partial adding

On Mon, 2011-12-12 at 12:25 +0800, Alex,Shi wrote:
> On Mon, 2011-12-12 at 12:35 +0800, Li, Shaohua wrote:
> > On Mon, 2011-12-12 at 12:14 +0800, Shi, Alex wrote:
> > > On Mon, 2011-12-12 at 10:43 +0800, Li, Shaohua wrote:
> > > > 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, 
> > > 
> > > The merge error that you fixed in 3.2-rc1 for hackbench regression is
> > > due to add slub to node partial head. And data of hackbench show node
> > > partial is still heavy used in allocation. 
> > The patch is already in base kernel, did you mean even with it you still
> > saw the list locking issue with latest kernel?
> > 
> 
> Yes, list_lock still hurt performance. It will be helpful if you can do
> some optimize for it. 
please post data and the workload. In my test, I didn't see the locking
takes significant time with perf. the slub stat you posted in last mail
shows most allocation goes the fast path.

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