Re: [RFC PATCH]vhost-blk: In-kernel accelerator for virtio blockdevice

[Liu Yuan]

On 08/01/2011 04:17 PM, Avi Kivity wrote:
> On 07/29/2011 06:25 PM, Sasha Levin wrote:
> > On Fri, 2011-07-29 at 20:01 +0800, Liu Yuan wrote:
> > >  Looking at this long list,most are function pointers that can not be
> > >  inlined, and the internal data structures used by these functions are
> > >  dozons. Leave aside code complexity, this long code path would really
> > >  need retrofit. As Christoph simply put, this kind of mess is inherent
> > >  all over the qemu code. So I am afraid, the 'retrofit'  would end 
> > up to
> > >  be a re-write the entire (sub)system. I have to admit that, I am
> > >  inclined to the MST's vhost approach, that write a new subsystem 
> > other
> > >  than tedious profiling and fixing, that would possibly goes as far as
> > >  actually re-writing it.
> >
> > I don't think the fix for problematic userspace is to write more kernel
> > code.
> >
> > vhost-net improved throughput and latency by several factors, allowing
> > to achieve much more than was possible at userspace alone.
> >
> > With vhost-blk we see an improvement of ~15% - which I assume by your
> > and Christoph's comments can be mostly attributed to QEMU. Merging a
> > module which won't improve performance dramatically compared to what is
> > possible to achieve in userspace (even if it would require a code
> > rewrite) sounds a bit wrong to me
>
> Agree.  vhost-net works around the lack of async zero copy networking 
> interface.  Block I/O on the other hand does have such an interface, 
> and in addition transaction rates are usually lower.  All we're saving 
> is the syscall overhead.
Personally I too agree with Sasha Levin. But vhost-blk is the first fast 
prototype that is supposed to act as a code base to do further 
optimisation, which I plan to utilize  kernel's internal stuff like BIO 
layer,  that can not be accessed from user space, to maximize the 
performance for raw disk based block IO.

Yuan
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