Re: [dpdk-dev] performance degradation with fpic

[Thomas Monjalon <thomas@monjalon.net>]

02/11/2020 16:00, Bruce Richardson:
> On Mon, Nov 02, 2020 at 10:40:54AM +0000, Ali Alnubani wrote:
> > Hi Bruce,
> > 
> > I was able to pin this down on drivers/net/mlx5/mlx5_rxtx.c. Removing -fPIC from its ninja recipe in build.ninja resolves the issue (had to prevent creating shared libs in this case).
> > What do you suggest I do? Can we have per-pmd customized compilation flags?
> > 
> > Regards,
> > Ali
> > 
> There are multiple possible ways to achieve this, but below are some ideas:
> 
> 1. Take the changes for supporting function versioning and duplicate them
> from lib/meson.build to drivers/meson.build. Since function versioning
> support already requires everything to be built twice, we could set it to
> not use -fpic for the static libs in that case. Then mark mlx5 as using
> function versioning. This is a bit hackish though, so
> 
> 2. The "objs" parameter from each sub-directory is not widely used, so we
> could split this easily enough into objs-shared and objs-static, and allow
> the subdirectory build file, in this case mlx5/meson.ninja, to build any c
> files manually to pass them back. This is more flexible, and also means
> that you can limit the files which are to be built twice to only the single
> file, rather than marking the whole driver as needing rebuild.

Can it be done only in the driver?
No general meson change for this option?

> I'm sure there are other approaches too. However, I agree with Luca's
> comment that first approach should probably be to see if you can track down
> exactly why this one file is having problems. Could any of the slowdown be
> due to the fact that you use a common lib from your driver? Are there
> cross-driver calls in the fast-path that are suffering a penalty?

Of course the performance will be analyzed in the long run.
However, such analyzis is more convenient if meson is flexible enough
to allow customization of the build.
And in general, I think it is good to have meson flexible
to allow any kind of driver build customization.