Re: [dpdk-dev] [PATCH] net/af_xdp: optimisations to improve packet loss

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri, 12 Jun 2020 14:17:46 +0000
Ciara Loftus <ciara.loftus@intel.com> wrote:

> This commit makes some changes to the AF_XDP PMD in an effort to improve
> its packet loss characteristics.
> 
> 1. In the case of failed transmission due to inability to reserve a tx
> descriptor, the PMD now pulls from the completion ring, issues a syscall
> in which the kernel attempts to complete outstanding tx operations, then
> tries to reserve the tx descriptor again. Prior to this we dropped the
> packet after the syscall and didn't try to re-reserve.
> 
> 2. During completion ring cleanup, always pull as many entries as possible
> from the ring as opposed to the batch size or just how many packets
> we're going to attempt to send. Keeping the completion ring emptier should
> reduce failed transmissions in the kernel, as the kernel requires space in
> the completion ring to successfully tx.
> 
> 3. Size the fill ring as twice the receive ring size which may help reduce
> allocation failures in the driver.
> 
> With these changes, a benchmark which measured the packet rate at which
> 0.01% packet loss could be reached improved from ~0.1G to ~3Gbps.
> 
> Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>

You might want to add the ability to emulate a tx_free threshold
by pulling more completions earlier.