linux-kernel - Re: [RFC] Unify KVM kernel-space and user-space code into a single project

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Message-ID: <20100321132709.GA7962@twister.home>
Date:	Sun, 21 Mar 2010 14:27:10 +0100
From:	Gabor Gombas <gombasg@...ikabel.hu>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Anthony Liguori <anthony@...emonkey.ws>,
	Avi Kivity <avi@...hat.com>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Sheng Yang <sheng@...ux.intel.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	oerg Roedel <joro@...tes.org>,
	Jes Sorensen <Jes.Sorensen@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>, ziteng.huang@...el.com,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Fr?d?ric Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC] Unify KVM kernel-space and user-space code into a single
 project

On Thu, Mar 18, 2010 at 05:13:10PM +0100, Ingo Molnar wrote:

> > Why does Linux AIO still suck?  Why do we not have a proper interface in 
> > userspace for doing asynchronous file system operations?
> 
> Good that you mention it, i think it's an excellent example.
> 
> The suckage of kernel async IO is for similar reasons: there's an ugly package 
> separation problem between the kernel and between glibc - and between the apps 
> that would make use of it.

No, kernel async IO sucks because it still does not play well with
buffered I/O. Last time I checked (about a year ago or so), AIO syscall
latencies were much worse when buffered I/O was used compared to direct
I/O. Unfortunately, to achieve good performance with direct I/O, you
need a HW RAID card with lots of on-board cache.

Gabor
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