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Date: Mon, 01 Jul 2013 16:33:44 -0700 From: Rick Jones <rick.jones2@...com> To: netdev@...r.kernel.org Subject: Re: a tap mystery On 06/10/2013 05:16 PM, Rick Jones wrote: > I have a small test script which runs a netperf TCP_RR test with an ever > increasing number of tap devices on the system. In this case the system > is a venerable Centrino-based laptop on AC power at fixed frequency, > with idle=poll for perf profiling purposes, the irqbalanced shot in the > head and the IRQ of the Intel Corporation 82566MM pointed at CPU 0, > whereon I have also bound the netperf process. The other system is my > personal workstation and the network connection between them is a > private, back-to-back link. The kernel on the laptop is a 3.5.0-30 > generic kernel. > > For the first 1024 tap devices created on the system, and put into the > "UP" state, what netperf reports for CPU utilization and service demand > is consistent with increasing per-packet costs which seems to be > consistent with list_for_each_entry_rcu(ptype, &ptype_all, list) usage > in dev_queue_xmit_nit and __netif_receive_skb. > > > But somewhere between 1024 and 2048 tap devices some sort of miracle > occurs and the CPU utilization and service demand drop. Considerably. And the same thing happens with an ~5 day old 3.10.0-rc6+ net-next kernel. happy benchmarking, rick jones -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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