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

[Peter Zijlstra <peterz@infradead.org>]

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.