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

["Zhang, Qi Z" <qi.z.zhang@intel.com>]

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

> >  	if (tx_rs_thresh >= (nb_desc - 2)) {
> >  		PMD_INIT_LOG(ERR, "tx_rs_thresh must be less than the "
> >  			     "number of TX descriptors minus 2. "
> > --
> > 2.16.0