Re: [patch 21/21] slab defrag: Obsolete SLAB

From: Zhang, Yanmin
Date: Fri May 16 2008 - 01:19:53 EST

On Thu, 2008-05-15 at 10:05 -0700, Christoph Lameter wrote:
> On Thu, 15 May 2008, Zhang, Yanmin wrote:
> > > It can thrash cachelines if objects from the same slab page are freed
> > > simultaneously on multiple processors. That occurred in the hackbench
> > > regression that we addressed with the dynamic configuration of slab sizes.
> > hackbench regression is because of slow allocation instead of slow freeing.
> > With ÃÃdynamic configuration of slab sizes, fast allocation becomes 97% (the bad
> > one is 68%), but fast free is always 8~9% with/without the patch.
> Thanks for using the slab statistics. I wish I had these numbers for the
> TPC benchmark. That would allow us to understand what is going on while it
> is running.
> The frees in the hackbench were slow because partial list updates occurred
> to frequently. The first fix was to let slab sit longer on the partial
> list.
I forgot that. 2.6.24 merged the patch.

> The other was the increase of the slab sizes which also increases
> the per cpu slab size and therefore the objects allocatable without a
> round trip to the page allocator.
That is what I am talking. 2.6.26-rc merged the patch.

> Freeing to a per cpu slab never requires
> partial list updates. So the frees also benefitted from the larger slab
> sizes. But the effect shows up in the count of partial list updates not in
> the fast/free collumn.
I agree. It might be better if SLUB could be optimized again to ïhave more consideration
when the slow free percentage is high, because the page lock might ping-pong
among processors if multi-processors access the same slab at the same time.

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