HEVC Encoding Performance / Driver 349.12

Hello,

Iam facing with a problem and I wanted to ask you:

I have tried NvEncoder and we noticed that it can not go over 35-36 fps for 4K HEVC video encoding.
Also, FFmpeg added NVENC API in their code including both H264 and H265 encoding features, and FFmpeg also can not pass the 35-36 fps limit for encoding a 4K YUV video to H265 using above cards.

My target is to encode 60fps 4K videos with Maxwell 2 GPUs. We thought GTX 980 is not enough to handle this. Over nvidia-smi, it is still not possible to see GPU usage for GTX 980.
But, nvidia-smi displays full info for Titan X. I can see that GPU Utilization does not pass 6-7% at all.

When I run 2 instances of 4K HEVC 30fps encoding and both work on 30 fps, which means that cards are actually can do total 60fps in 2 different commands.
However, on a single command, it looks like there is a limitation.

This is the value settings of HEVC Encoding:

ctx->init_encode_params.version = -804976880
ctx->init_encode_params.encodeWidth = 3840
ctx->init_encode_params.encodeHeight = 2160
ctx->init_encode_params.darWidth = 3840
ctx->init_encode_params.darHeight = 2160
ctx->init_encode_params.frameRateNum = 50
ctx->init_encode_params.frameRateDen = 1
ctx->init_encode_params.enableEncodeAsync = 0
ctx->init_encode_params.enablePTD = 1
ctx->init_encode_params.reportSliceOffsets = 0
ctx->init_encode_params.enableSubFrameWrite = 0
ctx->init_encode_params.enableExternalMEHints = 0
ctx->init_encode_params.privDataSize = 0
ctx->init_encode_params.enableExternalMEHints = 0
ctx->init_encode_params.maxEncodeWidth = 3840
ctx->init_encode_params.maxEncodeHeight = 2160

ctx->init_encode_params.gopLength = 50
ctx->init_encode_params.frameIntervalP = 1
ctx->init_encode_params.monoChromeEncoding = 0
ctx->init_encode_params.frameFieldMode = 1
ctx->init_encode_params.mvPrecision = 3

encodeConfig.level = 0
encodeConfig.tier = 0
encodeConfig.minCUSize = 2
encodeConfig.maxCUSize = 3
encodeConfig.useConstrainedIntraPred = 0
encodeConfig.disableDeblockAcrossSliceBoundary = 0
encodeConfig.outputBufferingPeriodSEI = 0
encodeConfig.outputPictureTimingSEI = 0
encodeConfig.outputAUD = 0
encodeConfig.enableLTR = 0
encodeConfig.disableSPSPPS = 0
encodeConfig.repeatSPSPPS = 1
encodeConfig.enableIntraRefresh = 0
encodeConfig.idrPeriod = 50
encodeConfig.intraRefreshPeriod = 0
encodeConfig.intraRefreshCnt = 0
encodeConfig.maxNumRefFramesInDPB = 1
encodeConfig.ltrNumFrames = 0
encodeConfig.vpsId = 0
encodeConfig.spsId = 0
encodeConfig.ppsId = 0
encodeConfig.sliceMode = 0
encodeConfig.sliceModeData = 0
encodeConfig.maxTemporalLayersMinus1 = 0

rc_param.constQP = 28
rc_param.averageBitRate = 24000000
rc_param.maxBitRate = 24000000
rc_param.vbvBufferSize = 96000000

Ican not figure out why these cards can not do 60 fps EHVC 4K encoding.

Can any body help or give a clue ?