Multi-threading with Rayon crate

This document describes how to use Rayon for parallel processing in TFHE-rs, detailing configurations for single and multi-client applications with code examples.

Rayon is a popular Rust crate that simplifies writing multi-threaded code. You can use Rayon to write multi-threaded TFHE-rs code. However, due to the specifications of Rayon and TFHE-rs, certain setups are necessary.

Single-client application

The problem

The high-level API requires to call set_server_key on each thread where computations need to be done. So a first attempt to use Rayon with TFHE-rs might look like this:

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' internals, this may create BorrowMutError.

Working example

The correct way is to call rayon::broadcast as follows:

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

For applications that need to operate concurrently on data from different clients and require each client to use multiple threads, you need to create separate Rayon thread pools:

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], see the example below:

// 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);
    })
}