linux-kernel - Re: Major regression on hackbench with SLUB

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Date:	Fri, 7 Dec 2007 13:58:10 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Christoph Lameter <clameter@....com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: Major regression on hackbench with SLUB


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> > Can you do one run with oprofile, and see exactly where the cost is? It
> > should hopefully be pretty darn obvious, considering your timing.

I kicked off my kernel tests which will take a few hours.
I run "make -j256" (4*nr_CPUS) 10 times at SCHED_OTHER and 10 times
at SCHED_FIFO (chrt -f 10) for both SLUB and SLAB.

This is automated, so I need to wait for it to finish before I can
continue other tests.

> The biggest cost of __slab_alloc() in my profile is the "slab_lock()", but
> that may not be the one that causes problems in a 64-cpu setup, so it
> would be good to have that verified.

I'll run oprofile as soon as the kernel build test is done.

> case which can trigger on NUMA. That's another potential explanation of
> why you'd see such a *huge* slowdown (ie if the whole node-match thing
> doesn't work out, slub just never gets the fast-case at all). That said,
> the number of places that actually pass a specific node to slub is very
> limited, so I suspect it's not the node matching. But just disabling that
> test in slab_alloc() might be one thing to test.

I'll try that after the oprofile.

Thanks,

-- Steve

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