On Thu, 21 Feb 2019 23:19:41 +0100 Daniel Borkmann <dan...@iogearbox.net> wrote:
> Recent work on XDP from Björn and Magnus additionally found that > manually transforming the XDP return code switch statement with > more than 5 cases into if-else combination would result in a > considerable speedup in XDP layer due to avoidance of indirect > calls in CONFIG_RETPOLINE enabled builds. On i40e driver with > XDP prog attached, a 20-26% speedup has been observed [0]. Aside > from XDP, there are many other places later in the networking > stack's critical path with similar switch-case processing. Rather > than fixing every XDP-enabled driver and locations in stack by > hand, it would be good to instead raise the limit where gcc would > emit expensive indirect calls from the switch under retpolines I'm very happy to see this. Thanks to Björn for finding, analyzing and providing hand-coded-if-else code that demonstrated the performance issue for XDP. But I do think this GCC case-values-threshold param is a better and more generic solution to the issue we observed and measured in XDP land. And hopefully other parts of the network stack and kernel will also benefit. Acked-by: Jesper Dangaard Brouer <bro...@redhat.com> Thanks for following up on this Daniel, -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
