Re: [dpdk-dev] Overriding rte_config.h

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, Nov 02, 2021 at 11:20:04AM +0000, Ananyev, Konstantin wrote:
> 
> > On Fri, Oct 29, 2021 at 09:48:30AM -0400, Ben Magistro wrote:
> > > With the transition to meson, what is the best way to provide custom values
> > > to parameters in rte_config.h?  When using makefiles, (from memory, I
> > > think) we used common_base as a template that was copied in as a
> > > replacement for defconfig_x86....  Our current thinking is to apply a
> > > locally maintained patch so that we can track custom values easier to the
> > > rte_config.h file unless there is another way to pass in an overridden
> > > value.  As an example, one of the values we are customizing is
> > > IP_FRAG_MAX_FRAG.
> > >
> > > Cheers,
> > >
> > There is no one defined way for overriding values in rte_config with the
> > meson build system, as values there are ones that should rarely need to be
> > overridden. If it's the case that one does need tuning, we generally want
> > to look to either change the default so it works for everyone, or
> > alternatively look to replace it with a runtime option.
> >
> > In the absense of that, a locally maintained patch may be reasonable. To
> > what value do you want to change MAX_FRAG? Would it be worth considering as
> > a newer default value in DPDK itself, since the current default is fairly
> > low?
> 
> That might be an option, with IP_FRAG_MAX_FRAG==8 it should be able
> to cover common jumbo frame size (9K) pretty easily.
> As a drawback default reassembly table size will double.

Maybe not. I'm not an expert in the library, but it seems the basic struct
used for tracking the packets and fragments is "struct ip_frag_pkt". Due to
the other data in the struct and the linked-list overheads, the actual size
increase when doubling MAX_FRAG from 4 to 8 is only 25%. According to gdb
on my debug build it goes from 192B to 256B.

> Even better would be to go a step further and rework lib/ip_frag
> to make it configurable runtime parameter.
>
Agree. However, that's not as quick a fix as just increasing the default
max segs value which could be done immediately if there is consensus on it.

/Bruce