jax（贾克斯）

JAX - Autograd and XLA (Accelerated Linear Algebra)

JAX is a framework that combines a NumPy-like API with a powerful system of composable function transformations: Grad (differentiation), Jit (compilation), Vmap (vectorization), and Pmap (parallelization).

When to Use

• High-performance scientific simulations requiring GPU/TPU acceleration.
• Custom machine learning research where PyTorch/TF abstractions are too restrictive.
• Calculating higher-order derivatives (Hessians, Jacobians) for optimization.
• Physics-informed machine learning and differentiable simulations.
• Automatic vectorization of functions (no more manual batching).
• Running the same code on CPU, GPU, and TPU without changes.

Reference Documentation

Official docs: https://jax.readthedocs.io/ GitHub: https://github.com/google/jax Search patterns: jax.numpy, jax.jit, jax.grad, jax.vmap, jax.random

Core Principles

Pure Functions (Immutability)

JAX is built on functional programming. All functions must be pure: they should not have side effects (like modifying a global variable) and must return the same output for the same input. JAX arrays are immutable.

XLA (Just-In-Time Compilation)

JAX uses XLA to compile and optimize Python/NumPy code into efficient machine code for specific hardware.

Manual PRNG Handling

Unlike NumPy, JAX requires explicit management of random state (keys) to ensure reproducibility in parallel environments.

Quick Reference

Installation

# CPU
pip install jax jaxlib

# GPU (Check documentation for specific CUDA versions)
pip install "jax[cuda12_pip]" -f https://storage.googleapis.com/jax-releases/jax_cuda_releases.html

Standard Imports

import jax
import jax.numpy as jnp
from jax import grad, jit, vmap, pmap, random

Basic Pattern - Differentiate and JIT

import jax.numpy as jnp
from jax import grad, jit

# 1. Define a pure function
def f(x):
    return jnp.sin(x) + x**2

# 2. Transform: Create a gradient function
df_dx = grad(f)

# 3. Transform: Compile for speed
f_fast = jit(f)

# 4. Use
val = f_fast(2.0)
slope = df_dx(2.0)

Critical Rules

✅ DO

• Use jax.numpy (jnp) - It mirrors NumPy but supports JAX transformations.
• Write Pure Functions - Ensure functions only depend on inputs and don't modify external state.
• Handle PRNG Keys Manually - Use key, subkey = random.split(key) for every random operation.
• Use vmap for Batching - Write code for a single sample and let JAX handle the batch dimension.
• Set static_argnums in JIT - If a JIT'ed function takes a non-array argument (like a string or integer used in a loop), mark it as static.
• In-place updates via.at - Since arrays are immutable, use x = x.at[idx].set(val).

❌ DON'T

• Use in-place updates - x[idx] = val will raise an error.
• Use standard numpy (np) - Standard NumPy arrays don't support JAX transformations.
• Use Side Effects - Don't use print() or modify global variables inside JIT-compiled functions.
• Forget to block_until_ready() - JAX uses asynchronous execution. For accurate timing, use result.block_until_ready().

Anti-Patterns (NEVER)

import jax.numpy as jnp
from jax import jit, random

# ❌ BAD: Modifying a global variable inside a function
counter = 0
@jit
def bad_func(x):
    global counter
    counter += 1 # ❌ Side effect! Will only run once during compilation
    return x * 2

# ❌ BAD: Standard NumPy random (not reproducible/parallel-safe)
# val = np.random.randn(5)

# ✅ GOOD: JAX PRNG
key = random.key(42)
val = random.normal(key, (5,))

# ❌ BAD: In-place assignment
# x[0] = 1.0

# ✅ GOOD: Functional update
x = jnp.zeros(5)
x = x.at[0].set(1.0)

Function Transformations

Grad (Differentiation)

def loss(params, x, y):
    pred = jnp.dot(x, params)
    return jnp.mean((pred - y)**2)

# Gradient of loss with respect to the 1st argument (params)
grads = grad(loss)(params, x, y)

# Higher-order: Hessian
hessian = jax.hessian(loss)(params, x, y)

Jit (Just-In-Time Compilation)

@jit
def complex_math(x):
    # This whole block is compiled into one XLA kernel
    y = jnp.exp(x)
    return jnp.sin(y) / jnp.sqrt(x)

# First call: Compiles (slow)
# Subsequent calls: Super fast

Vmap (Automatic Vectorization)

def model(params, x):
    return jnp.dot(params, x)

# model works on 1D x. How to apply to a 2D batch of X?
# in_axes=(None, 0): don't map params, map the 0th axis of x
batch_model = vmap(model, in_axes=(None, 0))

batch_preds = batch_model(params, X_batch)

Random Numbers (jax.random)

The State Management

key = random.key(0)

# Never reuse the same key!
key, subkey = random.split(key)
data = random.normal(subkey, (10,))

key, subkey = random.split(key)
noise = random.uniform(subkey, (10,))

Working with PyTrees

Handling complex data structures (Dicts, Lists, Tuples)

JAX transformations work on "PyTrees" — nested containers of arrays.

params = {'weights': jnp.ones((5,)), 'bias': 0.0}

def predict(p, x):
    return jnp.dot(x, p['weights']) + p['bias']

# grad and jit handle the dictionary automatically
grads = grad(predict)(params, x)

Practical Workflows

1. Differentiable Physics: Solving a Simple ODE

def system_dynamics(state, t):
    # Simple harmonic oscillator
    x, v = state
    dxdt = v
    dvdt = -0.5 * x
    return jnp.array([dxdt, dvdt])

def loss_fn(initial_state, target_x):
    # Simulate for 10 steps using simple Euler
    state = initial_state
    dt = 0.1
    for i in range(10):
        state += system_dynamics(state, i*dt) * dt
    return (state[0] - target_x)**2

# We can take the gradient of the simulation with respect to initial state!
optimize_initial_state = grad(loss_fn)

2. Parameter Sweep with vmap

def simulation(param):
    # Some complex computation
    return jnp.sum(jnp.linspace(0, param, 100))

# Parallelize simulation over a range of parameters
params = jnp.linspace(1, 10, 100)
results = vmap(simulation)(params)

3. Distributed Training with pmap

# pmap replicates the function across multiple GPUs
# (assuming 8 GPUs are available)
# x = jnp.zeros((8, 1024))
# results = pmap(jnp.sin)(x)

Performance Optimization

Static Arguments in JIT

If your function uses a loop based on an input value, that value must be static.

from functools import partial

@partial(jit, static_argnums=(1,))
def power_loop(x, n):
    for i in range(n):
        x = x * x
    return x

Avoid Python Control Flow

Prefer JAX control flow (cond, while_loop, fori_loop) for better XLA optimization.

from jax.lax import cond

def safe_divide(x, y):
    return cond(y == 0, lambda _: 0.0, lambda _: x / y, operand=None)

Common Pitfalls and Solutions

The "Tracer" Error

Inside JIT, JAX doesn't see actual numbers, it sees "Tracers".

# ❌ Problem:
# @jit
# def func(x):
#     if x > 0: return x # Error! JAX doesn't know x's value during compile

# ✅ Solution:
# Use jax.lax.cond or mark x as static_argnum

NaN Gradients

If your function has singularities (like sqrt(0)), gradients will be NaN.

# ✅ Solution: Add a small epsilon
def safe_sqrt(x):
    return jnp.sqrt(x + 1e-8)

Memory Leaks on GPU

JAX pre-allocates 90% of GPU memory by default.

# ✅ Solution: Set environment variable
import os
os.environ['XLA_PYTHON_CLIENT_PREALLOCATE'] = 'false'

Best Practices

1. Always use pure functions - No side effects, deterministic outputs
2. Manage PRNG keys explicitly - Split keys for every random operation
3. Use JIT for hot loops - Compile functions that are called repeatedly
4. Leverage vmap for batching - Write single-sample code, let JAX handle batches
5. Mark static arguments - Use static_argnums for non-array parameters in JIT
6. Use functional updates - Always use .at methods for array modifications
7. Profile before optimizing - Use jax.profiler to find bottlenecks
8. Handle device placement - Use jax.device_put() to control where arrays live
9. Test on CPU first - Debug on CPU, then scale to GPU/TPU
10. Understand compilation costs - First JIT call is slow, subsequent calls are fast

JAX is the ultimate playground for differentiable science. By treating math as a series of functional transformations, it unlocks speeds and complexities that were previously impossible in Python.