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crazyboycjr/PFC_headroom_formula.md

Created March 12, 2018 06:20
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PFC_headroom_formula.md
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PFC_headroom_formula.md

PFC headroom formula

According to the 3th page of the reference material, the following elements need to be taken into consideration

  • Maximum transmission unit (MTU) on the transimitting end of receiver R
  • Speed of Wire W
  • Transceiver latency (negligible) < 1280bytes
  • Response time of sender S
  • MTU on the transmitting end of sender S

The headroom should be the possible total length of bits transmitted in these five stages above.

For 100GbE, 100 meter twin-ax copper cable, QSFP+

MTU[R] + 100Gb/s * 476ns * 2 + 512 * 60bits + MTU[S]

which in bytes

MTU[S] + MTU[R] + 3840bytes + 130bytes per 100 meters per GbE

Speed of wire W is the only variable element in the definition of the receiver thresholds.

Reference

Priority Flow Control: Build Reliable Layer 2 Infrastructure

@crazyboycjr
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https://community.mellanox.com/docs/DOC-2623#jive_content_id_2_Why_do_we_recommend_17K_for_both_xoff_and_xon_for_PFC_Dont_we_need_hysteresis_here

@crazyboycjr
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  1. What are the recommended values for xon/xoff when setting PG as lossless?
    The general equation is to set reserved headroom size minus the xoff threshold to be equal to 2 * MTU+2 * link BW * link propagation time.

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https://community.mellanox.com/s/article/howto-tune-receive-buffers-on-mellanox-adapter-cards

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