Re: [Qemu-devel] [RFC PATCH] Makefile: Enable synchronized parallel output when possible

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jun 21, 2018 at 11:20:35PM -0300, Philippe Mathieu-Daudé wrote:
> The '--output-sync' option is available since GNU make 4.0.
> 
>   5.4.1 Output During Parallel Execution
> 
>   When running several recipes in parallel the output from each
>   recipe appears as soon as it is generated, with the result that
>   messages from different recipes may be interspersed, sometimes
>   even appearing on the same line. This can make reading the output
>   very difficult.
> 
>   To avoid this you can use the ‘--output-sync’ (‘-O’) option. This
>   option instructs make to save the output from the commands it
>   invokes and print it all once the commands are completed.
>   Additionally, if there are multiple recursive make invocations
>   running in parallel, they will communicate so that only one of
>   them is generating output at a time.
> 
>   There are four levels of granularity when synchronizing output,
>   specified by giving an argument to the option (e.g., ‘-Oline’
>   or ‘--output-sync=recurse’).
> 
>     none
>       This is the default: all output is sent directly as it is
>       generated and no synchronization is performed.
> 
>     line
>       Output from each individual line of the recipe is grouped
>       and printed as soon as that line is complete. If a recipe
>       consists of multiple lines, they may be interspersed with
>       lines from other recipes.
> 
>     target
>       Output from the entire recipe for each target is grouped and
>       printed once the target is complete. This is the default if
>       the --output-sync or -O option is given with no argument.
> 
>     recurse
>       Output from each recursive invocation of make is grouped and
>       printed once the recursive invocation is complete.
> 
>   Regardless of the mode chosen, the total build time will be the
>   same. The only difference is in how the output appears.
> 
> Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> ---
> Maybe 'line' is better for workstation and 'target/recurse' for
> unattended builds.
> 
> doc: https://www.gnu.org/software/make/manual/html_node/Parallel-Output.html
> ---
>  Makefile | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/Makefile b/Makefile
> index e46f2b625a..e94c687afe 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -367,6 +367,10 @@ DOCS=
>  endif
>  
>  SUBDIR_MAKEFLAGS=$(if $(V),,--no-print-directory --quiet) BUILD_DIR=$(BUILD_DIR)
> +ifeq ($(shell test $(firstword $(subst ., ,$(MAKE_VERSION))) -ge 4; echo $$?),0)
> +SUBDIR_MAKEFLAGS+=--output-sync=target
> +endif
> +
>  SUBDIR_DEVICES_MAK=$(patsubst %, %/config-devices.mak, $(TARGET_LIST))
>  SUBDIR_DEVICES_MAK_DEP=$(patsubst %, %-config-devices.mak.d, $(TARGET_LIST))

I'd probably just use -O. If they change the default it will probably be
to a better one.

> -- 
> 2.18.0.rc2