The effect of inlining

["Morten Brørup" <mb@smartsharesystems.com>]

+CC techboard

> From: Maxime Coquelin [mailto:maxime.coquelin@redhat.com]
> Sent: Friday, 29 March 2024 14.05
> 
> Hi Stephen,
> 
> On 3/29/24 03:53, Stephen Hemminger wrote:
> > On Thu, 28 Mar 2024 17:10:42 -0700
> > Andrey Ignatov <rdna@apple.com> wrote:
> >
> >>>
> >>> You don't need always inline, the compiler will do it anyway.
> >>
> >> I can remove it in v2, but it's not completely obvious to me how is
> it
> >> decided when to specify it explicitly and when not?
> >>
> >> I see plenty of __rte_always_inline in this file:
> >>
> >> % git grep -c '^static __rte_always_inline' lib/vhost/virtio_net.c
> >> lib/vhost/virtio_net.c:66
> >
> >
> > Cargo cult really.
> >
> 
> Cargo cult... really?
> 
> Well, I just did a quick test by comparing IO forwarding with testpmd
> between main branch and with adding a patch that removes all the
> inline/noinline in lib/vhost/virtio_net.c [0].
> 
> main branch: 14.63Mpps
> main branch - inline/noinline: 10.24Mpps

Thank you for testing this, Maxime. Very interesting!

It is sometimes suggested on techboard meetings that we should convert more inline functions to non-inline for improved API/ABI stability, with the argument that the performance of inlining is negligible.

I think this test proves that the sum of many small (negligible) performance differences it not negligible!

> 
> Andrey, thanks for the patch, I'll have a look at it next week.
> 
> Maxime
> 
> [0]: https://pastebin.com/72P2npZ0