linux-kernel - Re: [ANNOUNCE] Native Linux KVM tool v2

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Message-ID: <20110616080758.GB28449@elte.hu>
Date:	Thu, 16 Jun 2011 10:07:58 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	Alexander Graf <agraf@...e.de>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Prasad Joshi <prasadjoshi124@...il.com>,
	Avi Kivity <avi@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Sasha Levin <levinsasha928@...il.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Asias He <asias.hejun@...il.com>,
	Jens Axboe <jaxboe@...ionio.com>
Subject: Re: [ANNOUNCE] Native Linux KVM tool v2


* Pekka Enberg <penberg@...nel.org> wrote:

> Hi Ingo,
> 
> On Thu, Jun 16, 2011 at 10:24 AM, Ingo Molnar <mingo@...e.hu> wrote:
> >  - executing AIO in the vcpu thread eats up precious vcpu execution
> >   time: combined QCOW2 throughput would be limited by a single
> >   core's performance, and any time spent on QCOW2 processing would
> >   not be spent running the guest CPU. (In such a model we certainly
> >   couldnt do more intelligent, CPU-intense storage solutions like on
> >   the fly compress/decompress of QCOW2 data.)
> 
> Most image formats have optional on-the-fly 
> compression/decompression so we'd need to keep the current I/O 
> thread scheme anyway.

Yeah - although high-performance setups will probably not use that.

> > I'd only consider KAIO it if it provides some *real* measurable 
> > performance advantage of at least 10% in some important usecase. 
> > A few percent probably wouldnt be worth it.
> 
> I've only been following AIO kernel development from the sidelines 
> but I really haven't seen any reports of significant gains over 
> read()/write() from a thread pool. Are there any such reports?

I've measured such gains myself a couple of years ago, using an 
Oracle DB and a well-known OLTP benchmark, on a 64-way system.

I also profiled+tuned the kernel-side AIO implementation to be more 
scalable so i'm reasonably certain that the gains exist, and they 
were above 10%.

So the kaio gains existed back then but they needed sane userspace 
(POSIX AIO with signal notification sucks) and needed a well-tuned 
in-kernel implementation as well. (the current AIO code might have 
bitrotted)

Also, synchronous read()/write() [and scheduler() :-)] scalability 
improvements have not stopped in the past few years so the 
performance picture might have shifted in favor of a thread pool.

Thanks,

	Ingo
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