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

[Neil Horman <nhorman-2XuSBdqkA4R54TAoqtyWWQ@public.gmane.org>]

On Wed, Sep 17, 2014 at 11:01:40AM +0100, Bruce Richardson wrote:
> To improve performance by using bulk alloc or vectored RX routines, we
> need to set rx free threshold (rxfreet) value to 32, so make this the
> testpmd default.
> 
> Signed-off-by: Bruce Richardson <bruce.richardson-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> ---
>  app/test-pmd/testpmd.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/app/test-pmd/testpmd.c b/app/test-pmd/testpmd.c
> index 9f6cdc4..5751607 100644
> --- 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?