Re: [PATCH RESEND 5/5] vhost-blk: Add vhost-blk support

[Asias He]

On 07/18/2012 10:31 PM, Jeff Moyer wrote:
> Asias He <asias@xxxxxxxxxx> writes:
>
> > On 07/18/2012 03:10 AM, Jeff Moyer wrote:
> > > Asias He <asias@xxxxxxxxxx> writes:
> > >
> > > > vhost-blk is a in kernel virito-blk device accelerator.
> > > >
> > > > This patch is based on Liu Yuan's implementation with various
> > > > improvements and bug fixes. Notably, this patch makes guest notify and
> > > > host completion processing in parallel which gives about 60% performance
> > > > improvement compared to Liu Yuan's implementation.
> > >
> > > So, first off, some basic questions.  Is it correct to assume that you
> > > tested this with buffered I/O (files opened *without* O_DIRECT)?
> > >   I'm pretty sure that if you used O_DIRECT, you'd run into problems (which
> > > are solved by the patch set posted by Shaggy, based on Zach Brown's work
> > > of many moons ago).  Note that, with buffered I/O, the submission path
> > > is NOT asynchronous.  So, any speedups you've reported are extremely
> > > suspect.  ;-)
> >
> > I always used O_DIRECT to test this patchset. And I mostly used raw
> > block device as guest image. Is this the reason why I did not hit the
> > problem you mentioned. Btw, I do have run this patchset on image based
> > file. I still do not see problems like IO hangs.
>
> Hmm, so do the iovec's passed in point to buffers in userspace?  I
> thought they were kernel buffers, which would have blown up in
> get_user_pages_fast.

Yes. The iovec's passed in point to userspace buffers. ;-)

--
Asias


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