Re: [Qemu-devel] [PATCH] Optimize screendump

[Stefan Hajnoczi <stefanha@gmail.com>]

On Sun, Jun 19, 2011 at 3:54 PM, Avi Kivity <avi@redhat.com> wrote:
> When running kvm-autotest, fputc() is often the second highest (sometimes #1)
> function showing up in a profile.  This is due to fputc() locking the file
> for every byte written.
>
> Optimize by using fputc_unlocked().  Since the file is local to the caller,
> clearly no locking is needed.  According to the manual, _GNU_SOURCE is all
> that's needed for the function to be present.
>
> Signed-off-by: Avi Kivity <avi@redhat.com>
> ---
>  hw/vga.c |    6 +++---
>  1 files changed, 3 insertions(+), 3 deletions(-)

"The nonstandard *_unlocked() variants occur on a few Unix systems,
and are available in recent glibc. They should probably not be used."

I wonder if this will break non-Linux platforms.  Perhaps buffer an
entire row of pixels instead and only fwrite(3) at the end of the
outer loop.

Stefan