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RandomX configuration

RandomX has 45 customizable parameters (see table below). We recommend each project using RandomX to select a unique configuration to prevent network attacks from hashpower rental services.

These parameters can be modified in source file configuration.h.

parameter description default value
RANDOMX_ARGON_MEMORY The number of 1 KiB Argon2 blocks in the Cache 262144
RANDOMX_ARGON_ITERATIONS The number of Argon2d iterations for Cache initialization 3
RANDOMX_ARGON_LANES The number of parallel lanes for Cache initialization 1
RANDOMX_ARGON_SALT Argon2 salt "RandomX\x03"
RANDOMX_CACHE_ACCESSES The number of random Cache accesses per Dataset item 8
RANDOMX_SUPERSCALAR_LATENCY Target latency for SuperscalarHash (in cycles of the reference CPU) 170
RANDOMX_DATASET_BASE_SIZE Dataset base size in bytes 2147483648
RANDOMX_DATASET_EXTRA_SIZE Dataset extra size in bytes 33554368
RANDOMX_PROGRAM_SIZE The number of instructions in a RandomX program 256
RANDOMX_PROGRAM_ITERATIONS The number of iterations per program 2048
RANDOMX_PROGRAM_COUNT The number of programs per hash 8
RANDOMX_JUMP_BITS Jump condition mask size in bits 8
RANDOMX_JUMP_OFFSET Jump condition mask offset in bits 8
RANDOMX_SCRATCHPAD_L3 Scratchpad size in bytes 2097152
RANDOMX_SCRATCHPAD_L2 Scratchpad L2 size in bytes 262144
RANDOMX_SCRATCHPAD_L1 Scratchpad L1 size in bytes 16384
RANDOMX_FREQ_* (29x) Instruction frequencies multiple values

Not all of the parameters can be changed safely and most parameters have some contraints on what values can be selected (checked at compile-time).

Disclaimer: The compile-time checks only prevent obviously broken configurations. Passing the checks does not imply that the configuration is safe and will not cause crashes or other issues. We recommend that each non-standard configuration is thoroughly tested before being deployed.

RANDOMX_ARGON_MEMORY

This parameter determines the amount of memory needed in the light mode. Memory is specified in KiB (1 KiB = 1024 bytes).

Permitted values

Integer powers of 2 in the range 1 - 2097152.

Notes

Lower sizes will reduce the memory-hardness of the algorithm.

RANDOMX_ARGON_ITERATIONS

Determines the number of passes of Argon2 that are used to generate the Cache.

Permitted values

Any positive integer.

Notes

The time needed to initialize the Cache is proportional to the value of this constant.

RANDOMX_ARGON_LANES

The number of parallel lanes for Cache initialization.

Permitted values

Any positive integer.

Notes

This parameter determines how many threads can be used for Cache initialization.

RANDOMX_ARGON_SALT

Salt value for Cache initialization.

Permitted values

Any string of byte values.

Note

Every implementation should choose a unique salt value.

RANDOMX_CACHE_ACCESSES

The number of random Cache access per Dataset item.

Permitted values

Any integer greater than 1.

Notes

This value directly determines the performance ratio between the 'fast' and 'light' modes.

RANDOMX_SUPERSCALAR_LATENCY

Target latency for SuperscalarHash, in cycles of the reference CPU.

Permitted values

Integers in the range 1 - 10000.

Notes

The default value was tuned so that a high-performance superscalar CPU running at 2-4 GHz will execute SuperscalarHash in similar time it takes to load data from RAM (40-80 ns). Using a lower value will make Dataset generation (and light mode) more memory bound, while increasing this value will make Dataset generation (and light mode) more compute bound.

RANDOMX_DATASET_BASE_SIZE

Dataset base size in bytes.

Permitted values

Integer powers of 2 in the range 64 - 4294967296 (inclusive).

Note

This constant affects the memory requirements in fast mode. Some values are unsafe depending on other parameters. See Unsafe configurations.

RANDOMX_DATASET_EXTRA_SIZE

Dataset extra size in bytes.

Permitted values

Non-negative integer divisible by 64.

Note

This constant affects the memory requirements in fast mode. Some values are unsafe depending on other parameters. See Unsafe configurations.

RANDOMX_PROGRAM_SIZE

The number of instructions in a RandomX program.

Permitted values

Positive integers divisible by 8 in the range 8 - 32768 (inclusive).

Notes

Smaller values will make RandomX more DRAM-latency bound, while higher values will make RandomX more compute-bound. Some values are unsafe. See Unsafe configurations.

RANDOMX_PROGRAM_ITERATIONS

The number of iterations per program.

Permitted values

Any positive integer.

Notes

Time per hash increases linearly with this constant. Smaller values will increase the overhead of program compilation, while larger values may allow more time for optimizations. Some values are unsafe. See Unsafe configurations.

RANDOMX_PROGRAM_COUNT

The number of programs per hash.

Permitted values

Any positive integer.

Notes

Time per hash increases linearly with this constant. Some values are unsafe. See Unsafe configurations.

RANDOMX_JUMP_BITS

Jump condition mask size in bits.

Permitted values

Positive integers. The sum of RANDOMX_JUMP_BITS and RANDOMX_JUMP_OFFSET must not exceed 16.

Notes

This determines the jump probability of the CBRANCH instruction. The default value of 8 results in jump probability of 1/28 = 1/256. Increasing this constant will decrease the rate of jumps (and vice versa).

RANDOMX_JUMP_OFFSET

Jump condition mask offset in bits.

Permitted values

Non-negative integers. The sum of RANDOMX_JUMP_BITS and RANDOMX_JUMP_OFFSET must not exceed 16.

Notes

Since the low-order bits of RandomX registers are slightly biased, this offset moves the condition mask to higher bits, which are less biased. Using values smaller than the default may result in a slightly lower jump probability than the theoretical value calculated from RANDOMX_JUMP_BITS.

RANDOMX_SCRATCHPAD_L3

RandomX Scratchpad size in bytes.

Permitted values

Any integer power of 2. Must be larger than or equal to RANDOMX_SCRATCHPAD_L2.

Notes

The default value of 2 MiB was selected to match the typical cache/core ratio of desktop processors. Using a lower value will make RandomX more core-bound, while using larger values will make the algorithm more latency-bound. Some values are unsafe depending on other parameters. See Unsafe configurations.

RANDOMX_SCRATCHPAD_L2

Scratchpad L2 size in bytes.

Permitted values

Any integer power of 2. Must be larger than or equal to RANDOMX_SCRATCHPAD_L1.

Notes

The default value of 256 KiB was selected to match the typical per-core L2 cache size of desktop processors. Using a lower value will make RandomX more core-bound, while using larger values will make the algorithm more latency-bound.

RANDOMX_SCRATCHPAD_L1

Scratchpad L1 size in bytes.

Permitted values

Any integer power of 2. The minimum is 64 bytes.

Notes

The default value of 16 KiB was selected to be about half of the per-core L1 cache size of desktop processors. Using a lower value will make RandomX more core-bound, while using larger values will make the algorithm more latency-bound.

RANDOMX_FREQ_*

Instruction frequencies (per 256 instructions).

Permitted values

There is a total of 29 different instructions. The sum of frequencies must be equal to 256.

Notes

Making changes to the default values is not recommended. The only exceptions are the instruction pairs IROR_R/IROL_R, FADD_R/FSUB_R and FADD_M/FSUB_M, which are functionally equivalent. Example of a safe custom configuration:

||default|custom| |-|------|------|-| |RANDOMX_FREQ_IROR_R|8|5| |RANDOMX_FREQ_IROL_R|2|5|

default custom
RANDOMX_FREQ_FADD_R 16 17
RANDOMX_FREQ_FSUB_R 16 15
default custom
RANDOMX_FREQ_FADD_M 5 4
RANDOMX_FREQ_FSUB_M 5 6

Unsafe configurations

There are some configurations that are considered 'unsafe' because they affect the security of the algorithm against attacks. If the conditions listed below are not satisfied, the configuration is unsafe and a compilation error is emitted when building the RandomX library.

These checks can be disabled by definining RANDOMX_UNSAFE when building RandomX, e.g. by using -DRANDOMX_UNSAFE command line switch in GCC or MSVC. It is not recommended to disable these checks except for testing purposes.

1. Memory-time tradeoffs

Condition

RANDOMX_CACHE_ACCESSES * RANDOMX_ARGON_MEMORY * 1024 + 33554432 >= RANDOMX_DATASET_BASE_SIZE + RANDOMX_DATASET_EXTRA_SIZE

Configurations not satisfying this condition are vulnerable to memory-time tradeoffs, which enables efficient mining in light mode.

Solutions

  • Increase RANDOMX_CACHE_ACCESSES or RANDOMX_ARGON_MEMORY.
  • Decrease RANDOMX_DATASET_BASE_SIZE or RANDOMX_DATASET_EXTRA_SIZE.

2. Insufficient Scratchpad writes

Condition

(128 + RANDOMX_PROGRAM_SIZE * RANDOMX_FREQ_ISTORE / 256) * (RANDOMX_PROGRAM_COUNT * RANDOMX_PROGRAM_ITERATIONS) >= RANDOMX_SCRATCHPAD_L3

Configurations not satisfying this condition are vulnerable to Scratchpad size optimizations due to low amount of writes.

Solutions

  • Increase RANDOMX_PROGRAM_SIZE, RANDOMX_FREQ_ISTORE, RANDOMX_PROGRAM_COUNT or RANDOMX_PROGRAM_ITERATIONS.
  • Decrease RANDOMX_SCRATCHPAD_L3.

3. Program filtering strategies

Condition

RANDOMX_PROGRAM_COUNT > 1

Configurations not satisfying this condition are vulnerable to program filtering strategies.

Solution

  • Increase RANDOMX_PROGRAM_COUNT to at least 2.

4. Low program entropy

Condition

RANDOMX_PROGRAM_SIZE >= 64

Configurations not satisfying this condition do not have a sufficient number of instruction combinations.

Solution

  • Increase RANDOMX_PROGRAM_SIZE to at least 64.

5. High compilation overhead

Condition

RANDOMX_PROGRAM_ITERATIONS >= 400

Configurations not satisfying this condition have a program compilation overhead exceeding 10%.

Solution

  • Increase RANDOMX_PROGRAM_ITERATIONS to at least 400.
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