[Qemu-devel] Re: [RFC PATCH 05/20] Introduce put_vector() and get_vector to QEMUFile and qemu_fopen_ops().

[Yoshiaki Tamura]

Anthony Liguori wrote:
> On 04/22/2010 10:37 PM, Yoshiaki Tamura wrote:
> > Anthony Liguori wrote:
> > > On 04/21/2010 12:57 AM, Yoshiaki Tamura wrote:
> > > > QEMUFile currently doesn't support writev(). For sending multiple
> > > > data, such as pages, using writev() should be more efficient.
> > > >
> > > > Signed-off-by: Yoshiaki Tamura<address@hidden>
> > >
> > > Is there performance data that backs this up? Since QEMUFile uses a
> > > linear buffer for most operations that's limited to 16k, I suspect you
> > > wouldn't be able to observe a difference in practice.
> >
> > I currently don't have data, but I'll prepare it.
> > There were two things I wanted to avoid.
> >
> > 1. Pages to be copied to QEMUFile buf through qemu_put_buffer.
> > 2. Calling write() everytime even when we want to send multiple pages
> > at once.
> >
> > I think 2 may be neglectable.
> > But 1 seems to be problematic if we want make to the latency as small
> > as possible, no?
>
> Copying often has strange CPU characteristics depending on whether the
> data is already in cache. It's better to drive these sort of
> optimizations through performance measurement because changes are not
> always obvious.

I agree.