[dpdk-dev] [PATCH v2 4/7] net/mlx4: merge Tx path functions

[Matan Azrad]

Hi Nelio

> -----Original Message-----
> From: Nélio Laranjeiro [mailto:nelio.laranjeiro at 6wind.com]
> Sent: Thursday, October 26, 2017 4:44 PM
> To: Matan Azrad <matan at mellanox.com>
> Cc: Ophir Munk <ophirmu at mellanox.com>; Adrien Mazarguil
> <adrien.mazarguil at 6wind.com>; dev at dpdk.org; Thomas Monjalon
> <thomas at monjalon.net>; Olga Shern <olgas at mellanox.com>; Mordechay
> Haimovsky <motih at mellanox.com>
> Subject: Re: [dpdk-dev] [PATCH v2 4/7] net/mlx4: merge Tx path functions
> 
> On Thu, Oct 26, 2017 at 12:30:54PM +0000, Matan Azrad wrote:
> > Hi Nelio
> > Please see my comments below (3).
> >
> >
> > > -----Original Message-----
> > > From: Nélio Laranjeiro [mailto:nelio.laranjeiro at 6wind.com]
> > > Sent: Thursday, October 26, 2017 3:12 PM
> > > To: Matan Azrad <matan at mellanox.com>
> > > Cc: Ophir Munk <ophirmu at mellanox.com>; Adrien Mazarguil
> > > <adrien.mazarguil at 6wind.com>; dev at dpdk.org; Thomas Monjalon
> > > <thomas at monjalon.net>; Olga Shern <olgas at mellanox.com>;
> Mordechay
> > > Haimovsky <motih at mellanox.com>
> > > Subject: Re: [dpdk-dev] [PATCH v2 4/7] net/mlx4: merge Tx path
> > > functions
> > >
> > > On Thu, Oct 26, 2017 at 10:31:06AM +0000, Matan Azrad wrote:
> > > > Hi Nelio
> > > >
> > > > I think the memory barrier discussion is not relevant for this
> > > > patch (if it will be relevant I will create new one).
> > > > Please see my comments inline.
> > >
> > > It was not my single comment.  There is also useless code like
> > > having null segments in the packets which is not allowed on DPDK.
> >
> > Sorry, but I can't find comments in the previous mails.
> 
> You should search in the series,
> 
> > Moreover  this comment(first time I see it) is not relevant to this patch and
> asking something else.
> > All what this patch does is to merge 2 functions to prevent double
> > asking about WQ remain space...
> 
> Again in the series itself.
> 
> The point, this series embed 7 patches for "performance improvement",
> whereas the single improvement is avoiding to call an outside function by
> copy/pasting it into the PMD.
> In fact it will save few cycles, but this improvements could have been much
> more if the it was not a bare copy/paste.
> 

This simple merge improves 0.2MPPS in my setup.
If you have more improvements (other than reduce if statement) regarding this merge please suggest. 

> The real question is what is the improvement?  If the improvement is
> significant, it worse having this series, otherwise it does not as it may also
> bring some bugs which may be resolve from its original source whereas this
> one will remain.
> 

Each commit in this series improves performance - all of them improve performance significantly and brought us to our target.

By the way, I think series discussion should be in patch 0 :)

> > Remove memory\compiler barriers or dealing with null segments are not in
> the scope here.
> >
> > >
> > > > Regarding this specific patch, I didn't see any comment from you,
> > > > Are you agree with it?
> > > >
> > > > > -----Original Message-----
> > > > > From: Nélio Laranjeiro [mailto:nelio.laranjeiro at 6wind.com]
> > > > > Sent: Wednesday, October 25, 2017 10:50 AM
> > > > > To: Ophir Munk <ophirmu at mellanox.com>
> > > > > Cc: Adrien Mazarguil <adrien.mazarguil at 6wind.com>; dev at dpdk.org;
> > > > > Thomas Monjalon <thomas at monjalon.net>; Olga Shern
> > > > > <olgas at mellanox.com>; Matan Azrad <matan at mellanox.com>
> > > > > Subject: Re: [dpdk-dev] [PATCH v2 4/7] net/mlx4: merge Tx path
> > > > > functions
> > > > >
> > > > > On Tue, Oct 24, 2017 at 08:36:52PM +0000, Ophir Munk wrote:
> > > > > > Hi,
> > > > > >
> > > > > > On Tuesday, October 24, 2017 4:52 PM, Nélio Laranjeiro wrote:
> > > > > > >
> > > > > > > On Mon, Oct 23, 2017 at 02:21:57PM +0000, Ophir Munk wrote:
> > > > > > > > From: Matan Azrad <matan at mellanox.com>
> > > > > > > >
> > > > > > > > Merge tx_burst and mlx4_post_send functions to prevent
> > > > > > > > double asking about WQ remain space.
> > > > > > > >
> > > > > > > > This should improve performance.
> > > > > > > >
> > > > > > > > Signed-off-by: Matan Azrad <matan at mellanox.com>
> > > > > > > > ---
> > > > > > > >  drivers/net/mlx4/mlx4_rxtx.c | 353
> > > > > > > > +++++++++++++++++++++----------------------
> > > > > > > >  1 file changed, 170 insertions(+), 183 deletions(-)
> > > > > > >
> > > > > > > What are the real expectation you have on the remaining
> > > > > > > patches of the series?
> > > > > > >
> > > > > > > According to the comment of this commit log "This should
> > > > > > > improve performance" there are too many barriers at each
> > > > > > > packet/segment level to improve something.
> > > > > > >
> > > > > > > The point is, mlx4_burst_tx() should write all the WQE
> > > > > > > without any barrier as it is processing a burst of packets
> > > > > > > (whereas Verbs functions which may only process a single
> packet).
> > > > > >
> > > > > > > The lonely barrier which should be present is the one to
> > > > > > > ensure that all the host memory is flushed before triggering the Tx
> doorbell.
> > > > > > >
> > > > > >
> > > > > > There is a known ConnectX-3 HW limitation: the first 4 bytes
> > > > > > of every TXWBB (64 bytes chunks) should be written in a
> > > > > > reversed order (from last TXWBB to first TXWBB).
> > > > >
> > > > > This means the first WQE filled by the burst function is the doorbell.
> > > > > In such situation, the first four bytes of it can be written
> > > > > before leaving the burst function and after a write memory barrier.
> > > > >
> > > > > Until this first WQE is not complete, the NIC won't start
> > > > > processing the packets.  Memory barriers per packets becomes
> useless.
> > > >
> > > > I think this is not true, Since mlx4 HW can prefetch advanced
> > > > TXbbs if their first 4 bytes are valid in spite of the first WQE
> > > > is still not valid (please
> > > read the spec).
> > >
> > > A compiler barrier is enough on x86 to forbid the CPU to re-order
> > > the instructions, on arm you need a memory barrier, there is a macro
> > > in DPDK for that, rte_io_wmb().
> > >
> > We are also using compiler barrier here.
> >
> > > Before triggering the doorbell you must flush the case, this is the
> > > only place where the rte_wmb() should be used.
> > >
> >
> > We are also using memory barrier only for this reason.
> >
> > > > > It gives something like:
> > > > >
> > > > >  uint32_t tx_bb_db = 0;
> > > > >  void *first_wqe = NULL;
> > > > >
> > > > >  /*
> > > > >   * Prepare all Packets by writing the WQEs without the 4 first bytes of
> > > > >   * the first WQE.
> > > > >   */
> > > > >  for () {
> > > > >  	if (!wqe) {
> > > > > 		first_wqe = wqe;
> > > > > 		tx_bb_db = foo;
> > > > > 	}
> > > > >  }
> > > > >  /* Leaving. */
> > > > >  rte_wmb();
> > > > >  *(uin32_t*)wqe = tx_bb_db;
> > > > >  return n;
> > > > >
> > > >
> > > > I will take care to check if we can do 2 loops:
> > > > Write all  last 60B per TXbb.
> > > > Memory barrier.
> > > > Write all first 4B per TXbbs.
> > > >
> > > > > > The last 60 bytes of any TXWBB can be written in any order
> > > > > > (before writing the first 4 bytes).
> > > > > > Is your last statement (using lonely barrier) is in accordance
> > > > > > with this limitation? Please explain.
> > > > > >
> > > > > > > There is also too many cases handled which are useless in
> > > > > > > bursts
> > > > > situation,
> > > > > > > this function needs to be re-written to its minimal use case i.e.
> > > > > processing a
> > > > > > > valid burst of packets/segments and triggering at the end of
> > > > > > > the burst the
> > > > > Tx
> > > > > > > doorbell.
> > > > > > >
> > > > >
> > > > > Regards,
> > > > >
> > > > > --
> > > > > Nélio Laranjeiro
> > > > > 6WIND
> > >
> > > Regards,
> > >
> > > --
> > > Nélio Laranjeiro
> > > 6WIND
> 
> --
> Nélio Laranjeiro
> 6WIND