On Fri, 6 Oct 2023 14:15:00 +0200 Mattias Rönnblom <hof...@lysator.liu.se> wrote:
> On 2023-10-05 13:51, Bruce Richardson wrote: > > The event structure in DPDK is 16-bytes in size, and events are > > regularly passed as parameters directly rather than being passed as > > pointers. > > When are events passed by-value, rather than by-reference? There are no > such examples in the public eventdev API. > > To help compiler optimize correctly, we can explicitly request > > 16-byte alignment for events, which means that we should be able > > to do aligned vector loads/stores (e.g. with SSE or Neon) when working > > with those events. > > > > That change is both helping and sabotaging the optimizer's work. Now > every stack allocation needs to be 2-byte aligned - in DPDK code, and in > the application. > > The effect this change has on an eventdev app using DSW is a ~3 > cycle/event performance degradation on an AMD Zen 3 system, and a ~4 > cycle/event performance degradation on a Skylake-generation Intel CPU. > > What scenarios do you have in mind, where this change would improve the > generated code? Something where there are no unaligned loads available > in the ISA, or they are much slower than their aligned counterparts? > > When I looked into the same issue for the DPDK IP checksumming routines, > there basically were no such. Not that I could find. > > > Signed-off-by: Bruce Richardson <bruce.richard...@intel.com> Don't understand what the issue is. Isn't the event structure typically on stack. Adding padding there would not help.
