From mboxrd@z Thu Jan  1 00:00:00 1970
From: Gabor Gombas <gombasg@digikabel.hu>
Subject: Re: [RFC] Unify KVM kernel-space and user-space code into a single
 project
Date: Sun, 21 Mar 2010 14:27:10 +0100
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Cc: Anthony Liguori <anthony@codemonkey.ws>,
	Avi Kivity <avi@redhat.com>,
	"Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Sheng Yang <sheng@linux.intel.com>,
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To: Ingo Molnar <mingo@elte.hu>
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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