On 2023-04-17 14:52, Jerin Jacob wrote: > On Thu, Apr 13, 2023 at 12:24 PM Mattias Rönnblom > <mattias.ronnb...@ericsson.com> wrote: >> >> >> void >> rte_event_return_new_credits(...); >> >> Thoughts? > > I see the following cons on this approach. >
Does the use case in my original e-mail seem like a reasonable one to you? If yes, is there some way one could solve this problem with a clever use of the current Eventdev API? That would obviously be preferable. > # Adding multiple APIs in fast path to driver layer may not > performance effective solution. For event devices with a software-managed credit system, pre-allocation would be very cheap. And, if an application prefer to handle back pressure after-the-fact, that option is still available. > # At least for cnxk HW, credits are for device, not per port. So cnxk > HW implementation can not use this scheme. > DSW's credit pool is also per device, but are cached on a per-port basis. Does cnxk driver rely on the hardware to signal "new event" back pressure? (From the driver code it looks like that is the case.) > Alternative solution could be, adding new flag for > rte_enqueue_new_burst(), where drivers waits until credit is available > to reduce the application overhead > and support in different HW > implementations if this use case critical. > > #define RTE_EVENT_FLAG_WAIT_TILL_CREDIT_AVILABLE (UINT32_C(1) << 0) > > This solution only works if the event device is the only source of work for the EAL thread. That is a really nice model, but I wouldn't trust on that to always be the case. Also, there may be work that should only be performed, if the system is not under very high load. Credits being available, especially combined with a flexible new even threshold would be an indicator. Another way would be to just provide an API call that gave an indication of a particular threshold has been reached (or simply return an approximation of the number of in-flight events). Such a mechanism wouldn't be able to leave any guarantees, but could make a future enqueue operation very likely to succeed. >> >> Best regards, >> Mattias
