Use multi-threading using the rayon crate

rayon is a popular crate to easily write multi-threaded code in Rust.

It is possible to use rayon to write multi-threaded TFHE-rs code. However due to internal details of rayon and TFHE-rs, there is some special setup that needs to be done.

Single Client Application

The Problem

The high level api requires to call set_server_key on each thread where computations needs to be done. So a first attempt at using rayon with TFHE-rs might look like this:

use rayon::prelude::*;
use tfhe::prelude::*;
use tfhe::{ConfigBuilder, set_server_key, FheUint8, generate_keys};

fn main() {
    let (cks, sks) = generate_keys(ConfigBuilder::default());
    
    let xs = [
        FheUint8::encrypt(1u8, &cks),
        FheUint8::encrypt(2u8, &cks),
    ];

    let ys = [
        FheUint8::encrypt(3u8, &cks),
        FheUint8::encrypt(4u8, &cks),
    ];


    // set_server_key in each closure as they might be
    // running in different threads
    let (a, b) = rayon::join(
      || {
          set_server_key(sks.clone());
          &xs[0] + &ys[0]
      },
      || {
          set_server_key(sks.clone());
          &xs[1] + &ys[1]
      }
    );
}

However, due to rayon's work stealing mechanism and TFHE-rs's internals, this may create `BorrowMutError'.

Working Example

The correct way is to call rayon::broadcast

use rayon::prelude::*;
use tfhe::prelude::*;
use tfhe::{ConfigBuilder, set_server_key, FheUint8, generate_keys};

fn main() {
    let (cks, sks) = generate_keys(ConfigBuilder::default());
    
    // set the server key in all of the rayon's threads so that
    // we won't need to do it later
    rayon::broadcast(|_| set_server_key(sks.clone()));
    // Set the server key in the main thread
    set_server_key(sks);
    
    let xs = [
        FheUint8::encrypt(1u8, &cks),
        FheUint8::encrypt(2u8, &cks),
    ];

    let ys = [
        FheUint8::encrypt(3u8, &cks),
        FheUint8::encrypt(4u8, &cks),
    ];

    let (a, b) = rayon::join(
      || {
          &xs[0] + &ys[0]
      },
      || {
          &xs[1] + &ys[1]
      }
    );

    let a: u8 = a.decrypt(&cks);
    let b: u8 = b.decrypt(&cks);
    assert_eq!(a, 4u8);
    assert_eq!(b, 6u8);
}

Multi-Client Applications

If your application needs to operate on data from different clients concurrently, and that you want each client to use multiple threads, you will need to create different rayon thread pools

use rayon::prelude::*;
use tfhe::prelude::*;
use tfhe::{ConfigBuilder, set_server_key, FheUint8, generate_keys};

fn main() {
    let (cks1, sks1) = generate_keys(ConfigBuilder::default());
    let xs1 = [
        FheUint8::encrypt(1u8, &cks1),
        FheUint8::encrypt(2u8, &cks1),
    ];

    let ys1 = [
        FheUint8::encrypt(3u8, &cks1),
        FheUint8::encrypt(4u8, &cks1),
    ];

    let (cks2, sks2) = generate_keys(ConfigBuilder::default());
    let xs2 = [
        FheUint8::encrypt(100u8, &cks2),
        FheUint8::encrypt(200u8, &cks2),
    ];

    let ys2 = [
        FheUint8::encrypt(103u8, &cks2),
        FheUint8::encrypt(204u8, &cks2),
    ];

    let client_1_pool = rayon::ThreadPoolBuilder::new().num_threads(4).build().unwrap();
    let client_2_pool = rayon::ThreadPoolBuilder::new().num_threads(2).build().unwrap();
    
    client_1_pool.broadcast(|_| set_server_key(sks1.clone()));
    client_2_pool.broadcast(|_| set_server_key(sks2.clone()));
    
    let ((a1, b1), (a2, b2)) = rayon::join(|| {
        client_1_pool.install(|| {
            rayon::join(
                || {
                    &xs1[0] + &ys1[0]
                },
                || {
                    &xs1[1] + &ys1[1]
                }
            )
        })
    }, || {
        client_2_pool.install(|| {
            rayon::join(
                || {
                    &xs2[0] + &ys2[0]
                },
                || {
                    &xs2[1] + &ys2[1]
                }
            )
        })
    });
    
    let a1: u8 = a1.decrypt(&cks1);
    let b1: u8 = b1.decrypt(&cks1);
    assert_eq!(a1, 4u8);
    assert_eq!(b1, 6u8);

    let a2: u8 = a2.decrypt(&cks2);
    let b2: u8 = b2.decrypt(&cks2);
    assert_eq!(a2, 203u8);
    assert_eq!(b2, 148u8);
}

This can be useful if you have some rust #[test]

// Pseudo code
#[test]
fn test_1() {
    let pool = rayon::ThreadPoolBuilder::new().num_threads(4).build().unwrap();
    pool.broadcast(|_| set_server_key(sks1.clone()));
    pool.install(|| {
        let result = call_to_a_multithreaded_function(...);
        assert_eq!(result, expected_value);
    })
}

#[test]
fn test_2() {
    let pool = rayon::ThreadPoolBuilder::new().num_threads(4).build().unwrap();
    pool.broadcast(|_| set_server_key(sks1.clone()));
    pool.install(|| {
        let result = call_to_another_multithreaded_function(...);
        assert_eq!(result, expected_value);
    })
}