linux-kernel - Re: [patch] SLQB slab allocator (try 2)

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Date:	Tue, 27 Jan 2009 10:07:52 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Nick Piggin <npiggin@...e.de>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lin Ming <ming.m.lin@...el.com>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Subject: Re: [patch] SLQB slab allocator (try 2)

On Mon, 2009-01-26 at 12:22 -0500, Christoph Lameter wrote:
> On Mon, 26 Jan 2009, Peter Zijlstra wrote:
> 
> > Then again, anything that does allocation is per definition not bounded
> > and not something we can have on latency critical paths -- so on that
> > respect its not interesting.
> 
> Well there is the problem in SLAB and SLQB that they *continue* to do
> processing after an allocation. They defer queue cleaning. So your latency
> critical paths are interrupted by the deferred queue processing.

No they're not -- well, only if you let them that is, and then its your
own fault.

Remember, -rt is about being able to preempt pretty much everything. If
the userspace task has a higher priority than the timer interrupt, the
timer interrupt just gets to wait.

Yes there is a very small hardirq window where the actual interrupt
triggers, but all that that does is a wakeup and then its gone again.

>  SLAB has
> the awful habit of gradually pushing objects out of its queued (tried to
> approximate the loss of cpu cache hotness over time). So for awhile you
> get hit every 2 seconds with some free operations to the page allocator on
> each cpu. If you have a lot of cpus then this may become an ongoing
> operation. The slab pages end up in the page allocator queues which is
> then occasionally pushed back to the buddy lists. Another relatively high
> spike there.

Like Nick has been asking, can you give a solid test case that
demonstrates this issue?

I'm thinking getting git of those cross-bar queues hugely reduces that
problem.

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