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

[Thomas Monjalon <thomas.monjalon@6wind.com>]

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.

> 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.

Thoughts?

-- 
Thomas