On Fri, Apr 21, 2017 at 10:50:12AM -0400, Willem de Bruijn wrote: > >>> Maybe I was wrong, but according to Michael's comment it looks like he > >>> want > >>> check affinity_hint_set just for speculative tx polling on rx napi > >>> instead > >>> of disabling it at all. > >>> > >>> And I'm not convinced this is really needed, driver only provide affinity > >>> hint instead of affinity, so it's not guaranteed that tx and rx interrupt > >>> are in the same vcpus. > >> > >> You're right. I made the restriction broader than the request, to really > >> err > >> on the side of caution for the initial merge of napi tx. And enabling > >> the optimization is always a win over keeping it off, even without irq > >> affinity. > >> > >> The cycle cost is significant without affinity regardless of whether the > >> optimization is used. > > > > > > Yes, I noticed this in the past too. > > > >> Though this is not limited to napi-tx, it is more > >> pronounced in that mode than without napi. > >> > >> 1x TCP_RR for affinity configuration {process, rx_irq, tx_irq}: > >> > >> upstream: > >> > >> 1,1,1: 28985 Mbps, 278 Gcyc > >> 1,0,2: 30067 Mbps, 402 Gcyc > >> > >> napi tx: > >> > >> 1,1,1: 34492 Mbps, 269 Gcyc > >> 1,0,2: 36527 Mbps, 537 Gcyc (!) > >> 1,0,1: 36269 Mbps, 394 Gcyc > >> 1,0,0: 34674 Mbps, 402 Gcyc > >> > >> This is a particularly strong example. It is also representative > >> of most RR tests. It is less pronounced in other streaming tests. > >> 10x TCP_RR, for instance: > >> > >> upstream: > >> > >> 1,1,1: 42267 Mbps, 301 Gcyc > >> 1,0,2: 40663 Mbps, 445 Gcyc > >> > >> napi tx: > >> > >> 1,1,1: 42420 Mbps, 303 Gcyc > >> 1,0,2: 42267 Mbps, 431 Gcyc > >> > >> These numbers were obtained with the virtqueue_enable_cb_delayed > >> optimization after xmit_skb, btw. It turns out that moving that before > >> increases 1x TCP_RR further to ~39 Gbps, at the cost of reducing > >> 100x TCP_RR a bit. > > > > > > I see, so I think we can leave the affinity hint optimization/check for > > future investigation: > > > > - to avoid endless optimization (e.g we may want to share a single > > vector/napi for tx/rx queue pairs in the future) for this series. > > - tx napi is disabled by default which means we can do optimization on top. > > Okay. I'll drop the vi->affinity_hint_set from the patch set for now. I kind of like it, let's be conservative. But I'd prefer a comment near it explaining why it's there. -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization