Re: [Qemu-devel] [RFC PATCH 1/9] qapi/block-core: add option for io_uring

[Aarushi Mehta <mehta.aaru20@gmail.com>]

On Tue, 2019-05-21 at 19:39 -0500, Eric Blake wrote:
> On 5/21/19 6:52 PM, Aarushi Mehta wrote:
> > Signed-off-by: Aarushi Mehta <mehta.aaru20@gmail.com>
> 
> Sparse on the details. The subject line says what, but without a
> 'why'
> for how io_uring is different from existing aio options, it's hard to
> see why I'd want to use it. Do you have any benchmark numbers?

For peak performance, io_uring helps us get to 1.7M 4k IOPS with
polling. aio reaches a performance cliff much lower than that, at 608K.
If we disable polling, io_uring is able to drive about 1.2M IOPS for
the (otherwise) same test case.

More details, and the source for the above is at
http://kernel.dk/io_uring.pdf

> > ---
> >  qapi/block-core.json | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/qapi/block-core.json b/qapi/block-core.json
> > index 7ccbfff9d0..116995810a 100644
> > --- a/qapi/block-core.json
> > +++ b/qapi/block-core.json
> > @@ -2776,11 +2776,12 @@
> >  #
> >  # @threads:     Use qemu's thread pool
> >  # @native:      Use native AIO backend (only Linux and Windows)
> > +# @io_uring:    Use linux io_uring
> 
> Missing a '(since 4.1)' tag.
> 
> >  #
> >  # Since: 2.9
> >  ##
> >  { 'enum': 'BlockdevAioOptions',
> > -  'data': [ 'threads', 'native' ] }
> > +  'data': [ 'threads', 'native','io_uring' ] }
> 
> Missing space after ',' (not essential, but matching style is nice).
> Should the new element be defined conditionally, so that
> introspection
> only sees the new enum member when compiled for Linux?
> 
I'm not sure what would be the benefits of that? We already check for
Linux at configure, and this would reduce readability. We aren't doing
this for native.
>