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Subject: RE: [PATCH] mbuf: add mbuf physical address field to dynamic field
Date: Sun, 3 Jul 2022 09:31:01 +0200
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35D871A4@smartserver.smartshare.dk>
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Thread-Topic: [PATCH] mbuf: add mbuf physical address field to dynamic field
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References: <20220630094538.5b27d0c6@hermes.local>
 <20220701122434.1887783-1-sthotton@marvell.com>
From: =?iso-8859-1?Q?Morten_Br=F8rup?= <mb@smartsharesystems.com>
To: "Shijith Thotton" <sthotton@marvell.com>, <stephen@networkplumber.org>,
 "Bruce Richardson" <bruce.richardson@intel.com>, <olivier.matz@6wind.com>
Cc: <dev@dpdk.org>, <jerinj@marvell.com>, <thomas@monjalon.net>,
 "Honnappa Nagarahalli" <Honnappa.Nagarahalli@arm.com>
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> From: Shijith Thotton [mailto:sthotton@marvell.com]
> Sent: Friday, 1 July 2022 14.25
>=20
> >> If all devices are configured to run in IOVA mode as VA, physical
> >> address field of mbuf (buf_iova) won't be used.

Will some of the hardware vendors please comment on this: Has IOVA VA =
mode become common over time, or is it still an exotic bleeding edge =
feature?

If it has become common, we should let DPDK evolve accordingly, and =
consider PA (non-VA) mode legacy, treating it as such. Don't get stuck =
in the past.

> >> In such cases,
> buf_iova
> >> space is free to use as a dynamic field. So a new dynamic field
> member
> >> (dynfield2) is added in mbuf structure to make use of that space.
> >>
> >> A new mbuf flag RTE_MBUF_F_DYNFIELD2 is introduced to help identify
> the
> >> mbuf that can use dynfield2.
> >>
> >> Signed-off-by: Shijith Thotton <sthotton@marvell.com>
> >
> > This seems like a complex and potentially error prone way to do =
this.

Perhaps this optimization should be a compile time option instead?

> > What is the use case?
> >
>=20
> PCI drivers with the flag RTE_PCI_DRV_NEED_IOVA_AS_VA only works in
> IOVA mode as
> VA. buf_iova field of mbuf is not used by those PMDs and can be used =
as
> a
> dynamic area to save space.
>=20
> > How much of a performance gain?
>=20
> No change in performance.

Freeing up 8 bytes in the first mbuf cache line is a major improvement!

This could provide a significant performance gain for some applications, =
by moving private/dynamic mbuf fields from the second to the first cache =
line, thus avoiding to write to the second cache line in the =
application's first pipeline stage.