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

[Paolo Bonzini <pbonzini@redhat.com>]

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<mst@redhat.com>
>
> ---
>
> 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.  The 
way to go would probably be to merge QEMUFile and QEMUBufferedFile's two 
buffering layers, which also removes a copy.

Paolo