[dpdk-dev] [PATCH 1/1] mbuf: move pool pointer in first half

Jerin Jacob jerinjacobk at gmail.com
Mon Nov 9 09:27:26 CET 2020


On Mon, Nov 9, 2020 at 1:34 PM Thomas Monjalon <thomas at monjalon.net> wrote:
>
> 09/11/2020 06:18, Jerin Jacob:
> > On Sun, Nov 8, 2020 at 2:03 AM Thomas Monjalon <thomas at monjalon.net> wrote:
> > > 07/11/2020 20:05, Jerin Jacob:
> > > > On Sun, Nov 8, 2020 at 12:09 AM Thomas Monjalon <thomas at monjalon.net> wrote:
> > > > > 07/11/2020 18:12, Jerin Jacob:
> > > > > > On Sat, Nov 7, 2020 at 10:04 PM Thomas Monjalon <thomas at monjalon.net> wrote:
> > > > > > >
> > > > > > > The mempool pointer in the mbuf struct is moved
> > > > > > > from the second to the first half.
> > > > > > > It should increase performance on most systems having 64-byte cache line,
> > > > > >
> > > > > > > i.e. mbuf is split in two cache lines.
> > > > > >
> > > > > > But In any event, Tx needs to touch the pool to freeing back to the
> > > > > > pool upon  Tx completion. Right?
> > > > > > Not able to understand the motivation for moving it to the first 64B cache line?
> > > > > > The gain varies from driver to driver. For example, a Typical
> > > > > > ARM-based NPU does not need to
> > > > > > touch the pool in Rx and its been filled by HW. Whereas it needs to
> > > > > > touch in Tx if the reference count is implemented.
> > > >
> > > > See below.
> > > >
> > > > > >
> > > > > > > Due to this change, tx_offload is moved, so some vector data paths
> > > > > > > may need to be adjusted. Note: OCTEON TX2 check is removed temporarily!
> > > > > >
> > > > > > It will be breaking the Tx path, Please just don't remove the static
> > > > > > assert without adjusting the code.
> > > > >
> > > > > Of course not.
> > > > > I looked at the vector Tx path of OCTEON TX2,
> > > > > it's close to be impossible to understand :)
> > > > > Please help!
> > > >
> > > > Off course. Could you check the above section any share the rationale
> > > > for this change
> > > > and where it helps and how much it helps?
> > >
> > > It has been concluded in the techboard meeting you were part of.
> > > I don't understand why we restart this discussion again.
> > > I won't have the energy to restart this process myself.
> > > If you don't want to apply the techboard decision, then please
> > > do the necessary to request another quick decision.
> >
> > Yes. Initially, I thought it is OK as we have 128B CL, After looking
> > into Thomas's change, I realized
> > it is not good for ARM64 64B catchlines based NPU as
> > - A Typical  ARM-based NPU does not need to touch the pool in Rx and
> > its been filled by HW. Whereas it needs to
> > touch in Tx if the reference count is implemented.
>
> Small note: It is not true for all Arm platforms.

Yes. Generally, it is not specific to Arm platform. Any platform that
has HW accelerated mempool.

>
> > - Also it will be effecting exiting vector routines
> >
> > I request to reconsider the tech board decision.
>
>
>


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