Re: [Qemu-devel] [PATCH RFC] qemu-file: output data directly if possible

[Michael S. Tsirkin]

On Mon, Oct 10, 2011 at 09:42:51AM +0200, Paolo Bonzini wrote:
> On 10/10/2011 01:56 AM, Michael S. Tsirkin wrote:
> >qemu file currently always buffers up data before writing it out.
> >At least for memory this is probably not a good idea:
> >writing out to file would be cheaper. Let's do
> >that if we can, which should be the common case. If we can't, buffer.
> >
> >Signed-off-by: Michael S. Tsirkin<address@hidden>
> >
> >---
> >
> >Completely untested, this is just thinking aloud.
> >Shouldn't the below save us a data copy in the
> >common case, helping speed up migration?
> 
> The problem here is qemu_put_byte and friends, where the indirection
> of a function call would probably slow things down.  In the common
> case, qemu_put_byte is called a lot and f->buf_index would not be
> zero.

True, maybe the right thing to do is use a size cutoff,
avoid a copy if buffer is large enough.
I note the buffer in qemu file is 32K - is that
based on some specific measurements or just
a random large number? Any objections to making
it smaller, like 4K?

>  The way to go would probably be to merge QEMUFile and
> QEMUBufferedFile's two buffering layers, which also removes a copy.
> 
> Paolo

Yes, it does look sane. QEMUFile doesn't seem to ever be used without
QEMUBufferedFile - is that true?

-- 
MST