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

Gabor Gombas
2010-03-21T13:27:15Z

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