linux-kernel - Re: [benchmark] 1% performance overhead of paravirt

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Date:	Wed, 17 Jun 2009 11:56:28 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Pavel Machek <pavel@....cz>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ingo Molnar <mingo@...e.hu>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Avi Kivity <avi@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [benchmark] 1% performance overhead of paravirt_ops on native kernels

On Wed, Jun 17, 2009 at 11:40:16AM +0200, Pavel Machek wrote:
> Hi!
> 
> > > > > The "problem" is that you could in theory run out of kmap frames, since if 
> > > > > everybody does a kmap() in an interruptible context and you have lots and 
> > > > > lots of threads doing different pages, you'd run out. But that has nothing 
> > > > > to do with kmap_atomic(), which is basically limited to just the number of 
> > > > > CPU's and a (very small) level of nesting.
> > > > 
> > > > This could be avoided with an anti-deadlock pool. If a task
> > > > attempts a nested kmap and already holds a kmap, then give it
> > > > exclusive access to this pool until it releases its last
> > > > nested kmap.
> > > 
> > > We just sleep, waiting for somebody to release their. Again, that 
> > > obviously won't work in atomic context, but it's easy enough to just have 
> > > a "we need to have a few entries free" for the atomic case, and make it 
> > > busy-loop if it runs out (which is not going to happen in practice 
> > > anyway).
> > 
> > The really theoretical one (which Andrew likes complaining about) is
> > when *everybody* is holding a kmap and asking for another one ;)
> > But I think it isn't too hard to make a pool for that. And yes we'd
> 
> Does one pool help?

So long as only one process is allowed access to the pool at
one time, yes I think it solves it. It would probably never
even hit in practice, so synchronization overhead would not
matter.


> Now you can have '*everyone* is holding the kmaps and is asking for
> another one'.
> 
> You could add as many pools as maximum nesting level... Is there
> maximum nesting level?

Yes there are only a set number of kmap_atomic nesting levels,
so if you converted them all to kmap then it would be that + 1.
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