2nd/10_Wiki/Topics/Domain_Programming/AI_and_ML/Leaky-ReLU-and-Activations.md

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id	title	category	status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	verification_status	tags	raw_sources	last_reinforced	github_commit	tech_stack
wiki-2026-0508-leaky-relu-and-activations	Leaky ReLU and Activations	10_Wiki/Topics	verified	self	Activation Functions ReLU Family GELU SiLU Swish	none	A	0.9	applied	activation relu gelu silu swiglu deep-learning		2026-05-10	pending	language framework Python PyTorch

Leaky ReLU and Activations

매 한 줄

"매 activation = 비선형성". ReLU 계열이 base, Transformer는 GELU/SiLU/SwiGLU.

매 핵심

매 ReLU 계열

• ReLU: max(0, x). 빠름, dying ReLU 문제.
• Leaky ReLU: max(αx, x), α=0.01. 음수 작게 통과.
• PReLU: α 학습 가능 파라미터.
• ELU: x>0이면 x, 아니면 α(eˣ-1). 평균 0에 가까움.
• SELU: scaled ELU. self-normalizing (FC + lecun_normal init).

매 Smooth 계열

• GELU: x·Φ(x). BERT/GPT 표준. xerf 또는 tanh 근사.
• SiLU/Swish: x·σ(x). PaLM, EfficientNet.
• Mish: x·tanh(softplus(x)). YOLOv4.

매 Gated 계열 (FFN)

• GLU: (xW)⊗σ(xV). 정보 게이팅.
• SwiGLU: (xW)⊗SiLU(xV). LLaMA, PaLM FFN. 보통 hidden ×2/3 보정.
• GeGLU: GELU 변형.

매 Output 전용

• Sigmoid: 이진. saturation→gradient vanish.
• Softmax: multi-class probability.
• Tanh: [-1,1]. RNN, GAN generator.

매 직관

• ReLU: 빠르고 단순, but dead neurons
• GELU/SiLU: smooth, 0근처 비선형성↑, deep transformer에 유리
• SwiGLU: gating으로 expressiveness↑, 동일 param 대비 성능↑

💻 패턴

PyTorch built-ins

import torch.nn as nn, torch.nn.functional as F
# 클래스
nn.ReLU(); nn.LeakyReLU(0.01); nn.PReLU()
nn.ELU(alpha=1.0); nn.SELU()
nn.GELU(approximate="tanh"); nn.SiLU(); nn.Mish()
# 함수
F.relu(x); F.leaky_relu(x, 0.01); F.gelu(x); F.silu(x)

SwiGLU FFN (LLaMA-style)

class SwiGLU(nn.Module):
    def __init__(self, d, d_hidden):
        super().__init__()
        self.w_gate = nn.Linear(d, d_hidden, bias=False)
        self.w_up   = nn.Linear(d, d_hidden, bias=False)
        self.w_down = nn.Linear(d_hidden, d, bias=False)
    def forward(self, x):
        return self.w_down(F.silu(self.w_gate(x)) * self.w_up(x))
# 보통 d_hidden = int(8/3 * d), 64 multiple로 round

GELU 직접

import math, torch
def gelu_tanh(x):
    return 0.5 * x * (1 + torch.tanh(math.sqrt(2/math.pi) * (x + 0.044715 * x**3)))

Init과 페어링

# ReLU/Leaky → He (Kaiming)
nn.init.kaiming_normal_(layer.weight, nonlinearity="relu")
# SELU → LeCun normal
nn.init.normal_(layer.weight, std=(1.0 / fan_in)**0.5)
# Tanh → Xavier
nn.init.xavier_normal_(layer.weight)

Dying ReLU 진단

# forward hook으로 zero ratio 측정
zero_ratio = (act == 0).float().mean().item()
# > 0.5 지속 → Leaky/ELU/GELU로 교체

매 결정 기준

모델	Activation
CNN classic	ReLU
ResNet/EfficientNet	ReLU / SiLU
Transformer (BERT/GPT)	GELU
LLaMA / PaLM FFN	SwiGLU
GAN generator	Tanh (out), ReLU (hidden)
Self-normalizing FC	SELU + lecun_normal
YOLO 변형	Mish
Output binary	Sigmoid
Output multiclass	Softmax (or none + CE)

기본값: 일반 DL → ReLU. Transformer → GELU. LLM FFN → SwiGLU.

🔗 Graph

• 부모: Neural-Networks, Deep Learning
• 변형: GELU
• 응용: Transformer, CNN
• Adjacent: Loss-Functions-Foundations

🤖 LLM 활용

언제: 모델별 표준 activation 추천, 코드 생성. 언제 X: 새로운 SoTA activation 검증은 실험 필요.

❌ 안티패턴

• ReLU + softmax 출력 hidden에 Sigmoid 끼우기
• SELU에 BatchNorm 같이 쓰기 (self-norm 깨짐)
• Sigmoid를 deep network hidden에 (vanishing)
• SwiGLU 쓰면서 hidden dim 보정 안 함 (param 늘어남)
• Output에 ReLU (negative target 못 표현)
• He init을 GELU/SiLU에도 (괜찮지만 정확히는 다름)

🧪 검증 / 중복

• Verified (He 2015, Hendrycks GELU, Ramachandran Swish, Shazeer SwiGLU). 신뢰도 A.
• 중복: 없음.

🕓 Changelog

날짜	변경
2026-05-08	Phase 1
2026-05-10	Manual cleanup — SwiGLU/GELU 코드, init pairing