thodg/cgminer/SCRYPT-README

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• Hash : b406dbfe
  Author :
  Date : 2012-08-07T20:07:01
  

  Update SCRYPT README with information about HW errors.

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• SCRYPT-README

• If you wish to donate to the author, Con Kolivas, in LTC, please submit your
  donations to:
  
  Lc8TWMiKM7gRUrG8VB8pPNP1Yvt1SGZnoH
  
  Otherwise, please donate in BTC as per the main README.
  
  ---
  
  
  Scrypt mining, AKA litecoin mining, for GPU is completely different to sha256
  used for bitcoin mining. The algorithm was originally developed in a manner
  that it was anticipated would make it suitable for mining on CPU but NOT GPU.
  Thanks to some innovative work by Artforz and mtrlt, this was proven to be
  wrong. However, it has very different requirements to bitcoin mining and is a
  lot more complicated to get working well. Note that it is a ram dependent
  workload, and requires you to have enough system ram as well as fast enough
  GPU ram. If you have less system ram than your GPU has, it may not be possible
  to mine at any reasonable rate.
  
  There are 5 main parameters to tuning scrypt, 2 of which you MUST set, and
  the others are optional for further fine tuning. When you start scrypt mining
  with the --scrypt option, cgminer will fail IN RANDOM WAYS. They are all due
  to parameters being outside what the GPU can cope with. Not giving cgminer a
  hint as to your GPU type, it will hardly ever perform well.
  
  NOTE that if it does not fail at startup, the presence of hardware errors (HW)
  are a sure sign that you have set the parameters too high.
  
  
  Step 1 on linux:
  export GPU_MAX_ALLOC_PERCENT=100
  If you do not do this, you may find it impossible to scrypt mine. You may find
  a value of 40 is enough and increasing this further has little effect.
  
  export GPU_USE_SYNC_OBJECTS=1
  may help CPU usage a little as well.
  
  --shaders XXX
  
  is a new option where you tell cgminer how many shaders your GPU has. This
  helps cgminer try to choose some meaningful baseline parameters. Use this table
  below to determine how many shaders your GPU has, and note that there are some
  variants of these cards, and nvidia shaders are much much lower and virtually
  pointless trying to mine on.
  
  GPU  Shaders
  7750 512
  7770 640
  7850 1024
  7870 1280
  7950 1792
  7970 2048
  
  6850 960
  6870 1120
  6950 1408
  6970 1536
  6990 (6970x2)
  
  6570 480
  6670 480
  6790 800
  
  6450 160
  
  5670 400
  5750 720
  5770 800
  5830 1120
  5850 1440
  5870 1600
  5970 (5870x2)
  
  These are only used as a rough guide for cgminer, and it is rare that this is
  all you will need to set.
  
  
  --intensity XX
  
  Just like in bitcoin mining, scrypt mining takes an intensity, however the
  scale goes from 0 to 20 to mimic the "Aggression" used in mtrlt's reaper. The
  reason this is crucial is that too high an intensity can actually be
  disastrous with scrypt because it CAN run out of ram. Intensities over 13
  start writing over the same ram and it is highly dependent on the GPU, but they
  can start actually DECREASING your hashrate, or even worse, start producing
  garbage with HW errors skyrocketing. The low level detail is that intensity is
  only guaranteed up to the power of 2 that most closely matches the thread
  concurrency. i.e. a thread concurrency of 6144 has 8192 as the nearest power
  of two above it, thus as 2^13=8192, that is an intensity of 13.
  
  
  Optional parameters to tune:
  -g, --thread-concurrency, --lookup-gap
  
  -g:
  Once you have found the optimal shaders and intensity, you can start increasing
  the -g value till cgminer fails to start. Rarely will you be able to go over
  about -g 4 and each increase in -g only increases hashrate slightly.
  
  --thread-concurrency:
  This tunes the optimal size of work that scrypt can do. It is internally tuned
  by cgminer to be the highest reasonable multiple of shaders that it can
  allocate on your GPU. Ideally it should be a multiple of your shader count.
  vliw5 architecture (R5XXX) would be best at 5x shaders, while VLIW4 (R6xxx and
  R7xxx) are best at 4x. Setting thread concurrency overrides anything you put
  into --shaders.
  
  --lookup-gap
  This tunes a compromise between ram usage and performance. Performance peaks
  at a gap of 2, but increasing the gap can save you some GPU ram, but almost
  always at the cost of significant loss of hashrate. Setting lookup gap
  overrides the default of 2, but cgminer will use the --shaders value to choose
  a thread-concurrency if you haven't chosen one.
  
  
  Overclocking for scrypt mining:
  First of all, do not underclock your memory initially. Scrypt mining requires
  memory speed and on most, but not all, GPUs, lowering memory speed lowers
  mining performance.
  
  Second, absolute engine clock speeds do NOT correlate with hashrate. The ratio
  of engine clock speed to memory matters, so if you set your memory to the
  default value, and then start overclocking as you are running it, you should
  find a sweet spot where the hashrate peaks and then it might actually drop if
  you increase the engine clock speed further. Unless you wish to run with a
  dynamic intensity, do not go over 13 without testing it while it's running to
  see that it increases hashrate AND utility WITHOUT increasing your HW errors.
  
  
  Suggested values for 7970 for example:
  export GPU_MAX_ALLOC_PERCENT=100
  --thread-concurrency 8192 -g 4 --gpu-engine 1135 --gpu-memclock 1375
  
  
  ---
  
  If you wish to donate to the author, Con Kolivas, in LTC, please submit your
  donations to:
  
  Lc8TWMiKM7gRUrG8VB8pPNP1Yvt1SGZnoH
  
  Otherwise, please donate in BTC as per the main README.
  

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