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- 1. Chainer v3 Chainer Meetup #06 @ PFN, Sep. 30, 2017 Seiya Tokui @ Preferred Networks
- 2. Recent/coming releases • Chainer v3.0.0 RC, v2.1.0: Sep. 12 • v3 RC was the 50th release! • CuPy v2.0.0 RC, v1.0.3 on the same day • Next release: Chainer v3.0.0 and v4.0.0α on Oct. 17 • CuPy v2.0.0 and v3.0.0α on the same day • Today, I mainly talk about the features of CuPy v2.0.0 RC and Chainer v3.0.0 RC
- 3. Chainer v3.0.0rc1 • For most users, the backward compatibility is maintained • See the release notes of v3.0.0rc1 for some small breaks that do not affect most users • The inner-working is greatly changed • It may cause some existing code that directly touches the computational graphs broken • Thanks to this change, we now support double backprop (a.k.a. gradient of gradients) as announced
- 4. Double backprop • Automatic backpropagation through gradients • When is it needed? • Consider a loss function that includes a gradient computation as a term/factor • E.g. the loss function for WGAN-GP: 𝔼 𝑥∼ℙ 𝑔 𝐷 𝑥 − 𝔼 𝑥∼ℙ 𝑟 𝐷 𝑥 + 𝜆𝔼 𝑥∼ℙ 𝑥 𝛻𝑥 𝐷 𝑥 2 − 1 2 • To take the gradient of this loss function, we need to do backprop through 𝛻𝑥 𝐷( 𝑥), which itself we want to compute with backprop! gradient
- 5. Double backprop in Chainer v3 • Many functions now support double backprop • Those functions are rewritten to implement a new interface named FunctionNode (such functions are called new-style Functions) • backward() takes Variable instead of ndarray as grad_outputs and return values, which means backward() itself can be differentiated • Variable has now an attribute grad_var, which represents the gradient as a Variable (so that we can use it in the computational graph)
- 6. How to implement WGAN-GP 1. Using Variable.backward() x_tilde = generator(z) x_hat = x + u * (x_tilde – x) D(x_hat).backward(enable_double_backprop=True) # 1st diff gp = lambda * (x_hat.grad_var – 1) ** 2 loss = D(x_tilde) – D(x) + gp model.cleargrads() # to clear the 1st diff of params loss.backward() # 2nd diff
- 7. How to implement WGAN-GP 2. Using grad() x_tilde = generator(z) x_hat = x + u * (x_tilde – x) gx_hat, = chainer.grad([D(x_hat)], [x_hat], enable_double_backprop=True) # 1st diff gp = lambda * (gx_hat – 1) ** 2 loss = D(x_tilde) – D(x) + gp loss.backward() # 2nd diff This version is more efficient because grad() can skip the gradient computation for parameters (thus also we can drop cleargrads()).
- 8. New-style Function support • Most “standard” functions are now ported to the new-style interface: +, -, *, Convolution2D, Deconvolution2D, EmbedID, Linear, LSTM, BatchNormalization, sigmoid, relu, leaky_relu, softmax, log_softmax, tanh, exp, mean_squared_error, softmax_cross_entropy, dropout, layer_normalization, transpose, reshape, broadcast_to, sum, concat, __getitem__, etc… • We are still working on widening the double backprop support. Contributions are also welcome!!
- 9. Other features • Functions: layer_normalization, selu, arctan2, prod, NumPy-compatible matmul • Links: ChildSumTreeLSTM, NaryTreeLSTM, BatchRenormalization • Other new features: LeCunNormal, as_variable(), Variable.array, strict option of load_npz(), etc.
- 10. CuPy v2.0.0rc1 • Sparse matrix support • Complex number support • Improved memory allocator • Many new functions, esp. of linear algebra routines
- 11. Sparse matrix support • cupy.sparse --- the sparse matrix support with APIs compatible to scipy.sparse • CSR/CSC/COO and diagonal format • Basic arithmetics, matrix product, element indexing • Slicing along the major axis • Dense <-> Sparse conversion
- 12. Complex number support • CuPy now supports complex numbers! • Dtypes complex32, complex64, complex128 are now available • Routines related to complex numbers: angle, conj, imag, real
- 13. Linear algebra routines • Solvers, matrix inversion, determinant, eigenvalues, etc.: solve, tensorsolve, inv, pinv, det, slogdet, eigh, eigvalsh, matrix_rank • All under cupy.linalg namespace • einsum is also supported (thanks, @fukatani!) • Flexible tensor product/reduction based on Einstein convention
- 14. Improved memory allocator • The memory pool is greatly improved • It now uses “best-fit with coalescing” algorithm • The memory region is reused even if the size does not exactly match • It may also contribute to the speed improvement, thanks to the reduced number of reallocations • Example: the new seq2seq example originally uses all the memory of 12GB GPU, whose usage is reduced to 3GB, and also the execution time is reduced by appx. 25%.
- 15. Next versions • As you may know, we slightly changed the release policy again; the stable releases may now include some new features (thus v2.1.0 instead of v2.0.3). • v4 is scheduled based on our release policy: v4.0.0 will be three months after v3.0.0 (which will be mid Jan. if there is no delay). • The core features of v4 is not determined yet; let’s have discussions!

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