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“ PC  PC Latency measurements ”

“ PC  PC Latency measurements ”. G.Lamanna , R.Fantechi & J.Kroon (CERN) TDAQ WG – 8.9.2011. Introduction. Why the latency measurement is important for us?:

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“ PC  PC Latency measurements ”

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  1. “PCPCLatencymeasurements” G.Lamanna, R.Fantechi & J.Kroon (CERN) TDAQ WG – 8.9.2011

  2. Introduction • Why the latency measurement is important for us?: • Primitives transmission to the L0TP through ethernet (Low level protocol, possibility to have switches, small spread in latency…) • Data collection in L1 PCs. (which protocol? Routers? Network congestions? Reliability? Fast check at application levels?...) • L1 trigger distribution. (Small latency? Latency stability? Broadcasting? …) • Feedback to select computer and network HW • Select the transmission protocol

  3. Transmission protocol: UDP vs TCP? • TCP • GOOD: • provides reliable, ordereddelivery of a • stream of bytes. • Flowcontrol. • BAD: • heavyprotocol to be implemented in hardware, • each packet needs to be acknowledged and at high rate can become a problem. • The latency is intrinsically not stable. • The packets can be fractioned for transmission optimization reasons • UDP • GOOD: • simpleand fastprotocol, • easyto be implemented in hardware, • does not require big resources. • BAD: • no reliability, ordering, or data integrity • provides an unreliableservice and datagrams may arrive out of order.

  4. Test setup • The software tools for measure the latency provide averages and isn’t clear the contribution to the latency due to the sender and the receiver • HW approach: use the oscilloscope to measure the time difference in packet transmission PC #1 PC #2 ETH LPT LPT scope [see also R.Fantechi TDAQ 25.5.2011]

  5. Hardware used LKRPN0 PCATE GPU1

  6. Results: PCATE & GPU1 PCATE PCATE GPU1 GPU1

  7. Results: GPU1 & LKRPN0 LKRPN0 LKRPN0 GPU1 GPU1

  8. Results: LKRPN0 & PCATE LKRPN0 LKRPN0 PCATE PCATE

  9. Cross check with sockperf • In GPU1->PCATE the result is compatible with the HW measurements • In the PCATE->GPU1 we have a very big latency PCATE PCATE GPU1 GPU1

  10. GPU1 mother board architecture Processor 1 Processor 2 QPI QPI QPI Test 2 PCI-E x4 QUAD PORT GBE IOH IOH QPI Test 1 ESIx4 PCI-E x1 DUAL PORT GBE ICH10R

  11. “Schizophrenic” test Same PC, different NIC Same PC, Same NIC, different port GPU1 GPU1 GPU1 GPU1

  12. What we learned from this first studies? • Both the characteristics of the sender and the receiver are important for the latency • In particular the sender seems to play an important role using sockperf • The latency increases with the packet dimension • The frequency of the packet isn’t relevant until the “jump” • The “jump” happens at relatively high frequency rate (> 20 kHz) • The “jump” probably depends on the NIC/BUS/Chipset more than on CPU/Memory • The absolute value of the latency depends on the PCs setup and the packet dimension: • Minimum (~300 B packets): ~50 us (recvGPU1 or LKRPN0), ~60 us (recv PCATE) • Maximum (~1500B, no jumps if delay is >50us): ~70 us (recvGPU1 or LKRPN0), ~90 us (recvPCATE)

  13. To Do • Use the TALK board to go “faster” (the 10 MHz can’t be reached using PCs) • Identify the responsible of the “jumps”: • Same NIC on different PCs • Different NICs on same PC • Measure the latency with the TELL1 • Any suggestions is more than welcome!!! LKRPN0 PCATE GPU1

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