Exactness
Last updated
Last updated
One of the most common operations in Concrete is Table Lookups
(TLUs). TLUs are performed with an FHE operation called Programmable Bootstrapping
(PBS). PBS's have a certain probability of error, which, when triggered, result in inaccurate results.
Let's say you have the table:
And you perform a Table Lookup using 4
. The result you should get is lut[4] = 16
, but because of the possibility of error, you could get any other value in the table.
The probability of this error can be configured through the p_error
and global_p_error
configuration options. The difference between these two options is that, p_error
is for individual TLUs but global_p_error
is for the whole circuit.
If you set p_error
to 0.01
, for example, it means every TLU in the circuit will have a 99% chance of being exact with a 1% probability of error. If you have a single TLU in the circuit, global_p_error
would be 1% as well. But if you have 2 TLUs for example, global_p_error
would be almost 2% (1 - (0.99 * 0.99)
).
However, if you set global_p_error
to 0.01
, the whole circuit will have 1% probability of error, no matter how many Table Lookups are included.
If you set both of them, both will be satisfied. Essentially, the stricter one will be used.
By default, both p_error
and global_p_error
is set to None
, which results in a global_p_error
of 1 / 100_000
being used.
Feel free to play with these configuration options to pick the one best suited for your needs! See to learn how you can set a custom p_error
and/or global_p_error
.
Configuring either of those variables impacts computation time (compilation, keys generation, circuit execution) and space requirements (size of the keys on disk and in memory). Lower error probabilities would result in longer computation times and larger space requirements.