Nvidia Compatibility Bits For Three-way SLI Users

Discussion in 'General Discussion' started by otta56, Apr 2, 2014.

  1. otta56

    otta56 Registered

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    The default Nvidia SLI compatibility bits in the profile for rF2 exposed by Nvidia Inspector do not provide very good scaling for 3 card setups.

    Nvidia support responded to a query requesting either a 3 card specific profile or documentation about compatibility settings with the following: "Unfortunately we do not have any such SLI profile. Also we do not have any document related to decoding SLI bits.
    We apologize for the inconvenience caused."

    It is however, possible to improve the performance of 3 card setups beyond the performance offered by the stock compatibility bits. Results are as follows:

    Nvidia Inspector settings: rFactor2 Settings:

    View attachment 12207 View attachment 12208

    These settings were used with 3x Nvidia Titan cards. The AFR-SFR uses only two cards but results in an increase of a minimum fps increase @ Limerock from 24 fps -> 40 fps. The stock compatibility bits gave a minimum of 24 fps although all three cards were enabled. GPU utilization is also near maximum (99%) for 2 cards under AFR-SFR versus 55-65% per card for 3 cards using the stock compatibility bits.

    There are some indications in Nvidia technical documents that DX9 has a maximum of 3 frame buffers that can be addressed. This means that quad SLI installs cannot use AFR1 or AFR 2 since each card requires one frame buffer. The method Nvidia uses to get around this limitation is AFR of SFR. In this mode the driver creates groups of multiple GPUs that share the work for a given frame in SFR mode and then uses these groups (AFR Group) in Alternate Frame Rendering (AFR) mode. AFR groups can consist of any number of GPUs. This method also appears to work for 3 card SLI installs.

    For reference purposes, the various compatibility bit settings for different sli configurations for rF2 as broken out by Nvidia Inspector's bit editor are listed below:

    0x02402004 - Single GPU
    0x02402001 - 2 Way SLI AFR (Alternate Frame Rendering)
    0x02402002 -SFR (Split Srame Rendering) (needs a minimum of 2 cards)
    0x02402003 - AFR-SFR (AFR of SFR gets around the 3 frame buffer limit of DX9. Try this if using a Tri- or Quad- SLI setup)
    0x02402006 - 3 Way SLI AFR (This setting does not provide increased performance for a Tri-SLI setup. There seems to be a limitation in scaling judging by the in-game SLI indicator which freezes at a set level for all sli settings except AFR-SFR where the scaling is higher.)
    0x02402005 - 4 Way SLI AFR (This is the stock setting.)

    The bits are read from right to left so that the bit(s) controlling the SLI configuration in the compatibility bits are the first ones listed by the bit editor in Nvidia Inspector.

    0x02430000 appears to be the default for AFR2 according to the bit editor. It is not possible to draw any further conclusions on what the bits do due to the lack of documentation from Nvidia.

    The conclusion from all of this is that ISI and Nvidia need to work together to tune the executable and the existing profile for 3 Way SLI operation.

    In the meantime, the AFR-SFR settings may help some users of 3 and 4 way SLI configurations.

    Update:

    SLI compatibility bits 0x02432003 may give slightly higher frame rates when used with the "SLI_PREDEFINED_MODE_FORCE_AFR2" setting in the SLI section of the rfactor2 profile using Nvidia Inspector. This setting combines the default 0x02430000 AFR2 setting with the AFR-SFR SLI bit (3). Frame rates for the tracks previously tested now remain between 47-61 fps for the listed rfactor2 graphics settings.
     
    Last edited by a moderator: Apr 3, 2014

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