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

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Date:	Tue, 9 Jun 2009 14:42:01 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	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 Tue, Jun 09, 2009 at 02:25:29PM +0200, Ingo Molnar wrote:
> 
> * Nick Piggin <npiggin@...e.de> wrote:
> 
> > > and using atomic kmaps 
> > >    is fragile and error-prone. I think we still have a FIXME of a 
> > >    possibly triggerable deadlock somewhere in the core MM code ...
> > 
> > Not that I know of. I fixed the last long standing known one with 
> > the write_begin/write_end changes a year or two ago. It wasn't 
> > exactly related to kmap of the pagecache (but page fault of the 
> > user address in copy_from_user).
> 
> > > OTOH, highmem is clearly a useful hardware enablement feature 
> > > with a slowly receding upside and a constant downside. The 
> > > outcome is clear: when a critical threshold is reached distros 
> > > will stop enabling it. (or more likely, there will be pure 
> > > 64-bit x86 distros)
> > 
> > Well now lots of embedded type archs are enabling it... So the 
> > upside is slowly increasing again I think.
> 
> Sure - but the question is always how often does it show up on lkml? 
> Less and less. There might be a lot of embedded Linux products sold, 
> but their users are not reporting bugs to us and are not sending 
> patches to us in the proportion of their apparent usage.
> 
> And on lkml there's a clear downtick in highmem relevance.

Definitely. Probably it works *reasonably* well enough in the
end that embedded systems with reasonable highmem:lowmem ratio
probably will work OK. Sadly for them in a year or two they
probably get the full burden of carrying the crap ;) 


> > > Highmem simply enables a sucky piece of hardware so the code 
> > > itself has an intrinsic level of suckage, so to speak. There's 
> > > not much to be done about it but it's not a _big_ problem 
> > > either: this type of hw is moving fast out of the distro 
> > > attention span.
> > 
> > Yes but Linus really hated the code. I wonder whether it is 
> > generic code or x86 specific. OTOH with x86 you'd probably still 
> > have to support different page table formats, at least, so you 
> > couldn't rip it all out.
> 
> In practice the pte format hurts the VM more than just highmem. (the 
> two are inseparably connected of course)
> 
> I did this fork overhead measurement some time ago, using 
> perfcounters and 'perf':
> 
>   Performance counter stats for './fork':
> 
>         32-bit  32-bit-PAE     64-bit
>      ---------  ----------  ---------
>      27.367537   30.660090  31.542003  task clock ticks     (msecs)
> 
>           5785        5810       5751  pagefaults           (events)
>            389         388        388  context switches     (events)
>              4           4          4  CPU migrations       (events)
>      ---------  ----------  ---------
>                     +12.0%     +15.2%  overhead
> 
> So PAE is 12.0% slower (the overhead of double the pte size and 
> three page table levels), and 64-bit is 15.2% slower (the extra 
> overhead of having four page table levels added to the overhead of 
> double the pte size). [the pagefault count noise is well below the 
> systematic performance difference.]
> 
> Fork is pretty much the worst-case measurement for larger pte 
> overhead, as it has to copy around a lot of pagetables.
> 
> Larger ptes do not come for free and the 64-bit instructions do not 
> mitigate the cachemiss overhead and memory bandwidth cost.

No question about that... but you probably can't get rid of that
because somebody will cry about NX bit, won't they?

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