[dpdk-dev] [PATCH v3 1/7] net/mlx4: remove error flows from Tx fast path
Adrien Mazarguil
adrien.mazarguil at 6wind.com
Tue Oct 31 11:16:58 CET 2017
Hi Matan,
On Mon, Oct 30, 2017 at 06:11:31PM +0000, Matan Azrad wrote:
> Hi Adrien
>
> > -----Original Message-----
> > From: Adrien Mazarguil [mailto:adrien.mazarguil at 6wind.com]
> > Sent: Monday, October 30, 2017 4:23 PM
> > To: Matan Azrad <matan at mellanox.com>
> > Cc: dev at dpdk.org; Ophir Munk <ophirmu at mellanox.com>
> > Subject: Re: [PATCH v3 1/7] net/mlx4: remove error flows from Tx fast path
> >
> > On Mon, Oct 30, 2017 at 10:07:23AM +0000, Matan Azrad wrote:
> > > Move unnecessary error flows to DEBUG mode.
> > >
> > > Signed-off-by: Matan Azrad <matan at mellanox.com>
> > > Acked-by: Adrien Mazarguil <adrien.mazarguil at 6wind.com>
> >
> > I missed a couple of details while reviewing the original version, the first one
> > being mlx4_post_send()'s return value is still documented as updating
> > rte_errno in case of error, it's not the case anymore after this patch.
> >
> Good attention, Will be fixed in next version.
>
> > Please see below for the other one:
> >
> > > ---
> > > drivers/net/mlx4/mlx4_rxtx.c | 16 ++++++----------
> > > 1 file changed, 6 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/drivers/net/mlx4/mlx4_rxtx.c
> > > b/drivers/net/mlx4/mlx4_rxtx.c
> > <snip>
> > > /**
> > > @@ -510,8 +508,6 @@ struct pv {
> > > assert(max <= elts_n);
> > > /* Always leave one free entry in the ring. */
> > > --max;
> > > - if (max == 0)
> > > - return 0;
> > > if (max > pkts_n)
> > > max = pkts_n;
> > > for (i = 0; (i != max); ++i) {
> >
> > While minor, this change has nothing to do with this patch, right?
> >
> Yes you right, maybe it can be merged in patch 4/7.
>
> > I think it can slightly degrade an application performance as it removes the
> > guarantee that subsequent code only needs to be run if there is at least one
> > packet to process in case the TX ring is constantly full (SW faster than HW).
> >
>
> In case the TX ring is full, the loop condition should fail in the start and then return with 0 because the packet counter is 0.(more 2 checks)
> Since this case are less common (in my opinion) than at least 1 free space in ring, we can prevent this unnecessary check for all these common cases.
>
> Are you sure the 2 extra check important for performance in this empty case? Doesn't the application will call us again?
No, I don't think they're important to performance, like the changes from
patch 4/7, I'm not certain they actually make any difference. My suggestion
was mainly to leave it alone because of that. It's OK if you want to keep
and move it to 4/7.
--
Adrien Mazarguil
6WIND
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