Re: [dpdk-dev] [PATCH] net/i40e: update tx_free_threshold to improve zero copy performance

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

On Thu, Apr 12, 2018 at 12:20:07PM +0000, Zhang, Qi Z wrote:
> Hi Junjie:
> 
> > -----Original Message-----
> > From: Ananyev, Konstantin
> > Sent: Thursday, April 12, 2018 7:52 PM
> > To: Chen, Junjie J <junjie.j.chen@intel.com>; Xing, Beilei
> > <beilei.xing@intel.com>; Zhang, Qi Z <qi.z.zhang@intel.com>
> > Cc: dev@dpdk.org; Chen, Junjie J <junjie.j.chen@intel.com>; Chen@dpdk.org
> > Subject: RE: [dpdk-dev] [PATCH] net/i40e: update tx_free_threshold to
> > improve zero copy performance
> > 
> > 
> > 
> > > -----Original Message-----
> > > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Junjie Chen
> > > Sent: Thursday, April 12, 2018 6:32 AM
> > > To: Xing, Beilei <beilei.xing@intel.com>; Zhang, Qi Z
> > > <qi.z.zhang@intel.com>
> > > Cc: dev@dpdk.org; Chen, Junjie J <junjie.j.chen@intel.com>;
> > > Chen@dpdk.org
> > > Subject: [dpdk-dev] [PATCH] net/i40e: update tx_free_threshold to
> > > improve zero copy performance
> > >
> > > From: "Chen, Junjie" <junjie.j.chen@intel.com>
> > >
> > > When vhost backend works in dequeue zero copy mode, nic locks virtio's
> > > buffer until there is less or equal than tx_free_threshold buffer
> > > remain and then free number of tx burst buffer. This causes packets
> > > drop in virtio side and impacts zero copy performance. So we need to
> > > increase the tx_free_threshold to let nic free virtio's buffer as soon as
> > possible.
> > > Also we keep the upper limit to tx max burst size to ensure least
> > > performance impact on non zero copy.
> > 
> > Ok but why vhost app can't just use tx_queue_setup() to specify desired value
> > for tx_free_thresh?
> > Why instead we have to modify PMD to satisfy needs of one app?
> > Konstantin
> 
> I think the commit log could include the explanation that this change is proved not impact 
> driver's performance and it reduce total memory be locked by PMD Tx, so basically it benefit
> application that share the same mem pool overall, vhost dequeue zero copy is one of the example.
> 
> > 
> > >
> > > Signed-off-by: Chen, Junjie <junjie.j.chen@intel.com>
> > > ---
> > >  drivers/net/i40e/i40e_rxtx.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/drivers/net/i40e/i40e_rxtx.c
> > > b/drivers/net/i40e/i40e_rxtx.c index 56a854cec..d9569bdc9 100644
> > > --- a/drivers/net/i40e/i40e_rxtx.c
> > > +++ b/drivers/net/i40e/i40e_rxtx.c
> > > @@ -2039,6 +2039,8 @@ i40e_dev_tx_queue_setup(struct rte_eth_dev
> > *dev,
> > >  		tx_conf->tx_rs_thresh : DEFAULT_TX_RS_THRESH);
> > >  	tx_free_thresh = (uint16_t)((tx_conf->tx_free_thresh) ?
> > >  		tx_conf->tx_free_thresh : DEFAULT_TX_FREE_THRESH);
> > > +	if (tx_free_thresh < nb_desc - I40E_TX_MAX_BURST)
> > > +		tx_free_thresh = nb_desc - I40E_TX_MAX_BURST;
> 
> I think we'd better still allow application to set tx_free_thresh, since a small tx_free_thresh may still have benefit to let driver handle the first strike after device restarted
> So, nb_desc - I40E_TX_MAX_BURST can only be set when tx_conf->tx_rs_thresh = 0
> 
> Regards
> Qi
> 
+1 for just changing in this case.

/Bruce