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

[Neil Horman <nhorman@tuxdriver.com>]

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

> Thoughts?
> 
> -- 
> Thomas
>