Re: [dpdk-dev] [PATCH 3/5] testpmd: Change rxfreet default to 32

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

> -----Original Message-----
> From: Neil Horman [mailto:nhorman@tuxdriver.com]
> Sent: Friday, September 19, 2014 11:25 AM
> To: Richardson, Bruce
> Cc: Thomas Monjalon; dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH 3/5] testpmd: Change rxfreet default to 32
> 
> On Fri, Sep 19, 2014 at 09:18:26AM +0000, Richardson, Bruce wrote:
> > > -----Original Message-----
> > > From: Neil Horman [mailto:nhorman@tuxdriver.com]
> > > Sent: Thursday, September 18, 2014 7:09 PM
> > > To: Thomas Monjalon
> > > Cc: Richardson, Bruce; dev@dpdk.org
> > > Subject: Re: [dpdk-dev] [PATCH 3/5] testpmd: Change rxfreet default to 32
> > >
> > > On Thu, Sep 18, 2014 at 07:13:52PM +0200, Thomas Monjalon wrote:
> > > > 2014-09-18 15:53, Richardson, Bruce:
> > > > > > > --- a/app/test-pmd/testpmd.c
> > > > > > > +++ b/app/test-pmd/testpmd.c
> > > > > > > @@ -225,7 +225,7 @@ struct rte_eth_thresh tx_thresh = {
> > > > > > >  /*
> > > > > > >   * Configurable value of RX free threshold.
> > > > > > >   */
> > > > > > > -uint16_t rx_free_thresh = 0; /* Immediately free RX descriptors by
> > > default. */
> > > > > > > +uint16_t rx_free_thresh = 32; /* Refill RX descriptors once every 32
> > > packets
> > > > > > */
> > > > > > >
> > > > > >
> > > > > > Why 32?  Was that an experimentally determined value?
> > > > > > Does it hold true for all PMD's?
> > > > >
> > > > > This is primarily for the ixgbe PMD, which is right now the most
> > > > > highly tuned driver, but it works fine for all other ones too,
> > > > > as far as I'm aware.
> > > >
> > > > Yes, you are changing this value for all PMDs but you're targetting
> > > > only one.
> > > > These thresholds are dependent of the PMD implementation. There's
> > > > something wrong here.
> > > >
> > > I agree. Its fine to do this, but it does seem like the sample application
> > > should document why it does this and make note of the fact that other PMDs
> > > may
> > > have a separate optimal value.
> > >
> > > > > Basically, this is the minimum setting needed to enable either the
> > > > > bulk alloc or vector RX routines inside the ixgbe driver, so it's
> > > > > best made the default for that reason. Please see
> > > > > "check_rx_burst_bulk_alloc_preconditions()" in ixgbe_rxtx.c, and
> > > > > RX function assignment logic in "ixgbe_dev_rx_queue_setup()" in
> > > > > the same file.
> > > >
> > > > Since this parameter is so important, it could be a default value
> somewhere.
> > > >
> > > > I think we should split generic tuning parameters and tuning parameters
> > > > related to driver implementation or specific hardware.
> > > > Then we should provide some good default values for each of them.
> > > > At last, if needed, applications should be able to easily tune the
> > > > pmd-specific parameters.
> > > >
> > > I like this idea.  I've not got an idea of how much work it is to do so, but in
> > > principle it makes sense.
> > >
> > > Perhaps for the immediate need, since rte_eth_rx_queue_setup allows the
> > > config
> > > struct to get passed directly to PMDs, we can create a reserved value
> > > RTE_ETH_RX_FREE_THRESH_OPTIMAL, that instructs the pmd to select
> > > whatever
> > > threshold is optimal for its own hardware?
> > >
> > > Neil
> > >
> > Actually, looking at the code, I would suggest a couple of options, some of
> which may be used together.
> > 	1) we make NULL a valid value for the rxconf structure parameter to
> rte_eth_rx_queue_setup. There is little information in it that should really need
> to be passed in by applications to the drivers, and that would allow the drivers to
> be completely free to select the best options for their own operation.
> > 	2) As a companion to that (or as an alternative), we could also allow
> each driver to provide its own functions for rte_eth_get_rxconf_default, and
> rte_eth_get_txconf_default, to be used by applications that want to use known-
> good values for thresholds but also want to tweak one of the other values e.g.
> for rx, set the drop_en bit, and for tx set the txqflags to disable offloads.
> > 	3) Lastly, we could also consider removing the threshold and other not-
> generally-used values from the rxconf and txconf structures and make those
> removed fields completely driver-set values. Optionally, we could provide an
> alternate API to tune them, but I don't really see this being useful in most cases,
> and I'd probably omit it unless someone can prove a need for such APIs.
> >
> These all sound fairly reasonable to me.
> Neil

Further thinking seems to me like 1 doesn't really go very far, so it falls between 2 and 3. Any preference between them?

/Bruce