On Fri, 22 Feb 2019 at 08:32, Jesper Dangaard Brouer <[email protected]> wrote: > > On Thu, 21 Feb 2019 23:19:41 +0100 > Daniel Borkmann <[email protected]> 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 <[email protected]> > > Thanks for following up on this Daniel,
I definitely prefer a switch-statement over the if-else-messiness in this context. Thanks for doing the deep-dive, Daniel! FWIW, Acked-by: Björn Töpel <[email protected]> > -- > Best regards, > Jesper Dangaard Brouer > MSc.CS, Principal Kernel Engineer at Red Hat > LinkedIn: http://www.linkedin.com/in/brouer
