feat(gateway): Cache-Driven RPM Buffer

- buffer 公式从 baseRPM/5 改为 concurrency + maxSessions
  保留 baseRPM/5 作为 floor 向后兼容
- 粘性路径 fallback 新增 [StickyCacheMiss] 结构化日志
  reason: rpm_red / gate_check / session_limit / wait_queue_full / account_cleared
- session_limit 路径跳过 wait queue 重试（RegisterSession 拒绝无副作用）
- 典型配置 buffer 从 3 提升至 13，大幅减少高峰期 Prompt Cache Miss
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/internal/service/account.go

@@ -1727,22 +1727,47 @@ func (a *Account) GetRPMStrategy() string {
 }
 
 // GetRPMStickyBuffer 获取 RPM 粘性缓冲数量
-// tiered 模式下的黄区大小，默认为 base_rpm 的 20%（至少 1）
+// Cache-driven: buffer = concurrency + maxSessions（覆盖幽灵窗口 + 稳态会话需求）
+// floor = baseRPM / 5（向后兼容 maxSessions=0 且 concurrency=0 场景）
 func (a *Account) GetRPMStickyBuffer() int {
 	if a.Extra == nil {
 		return 0
 	}
+
+	// 手动 override 最高优先级
 	if v, ok := a.Extra["rpm_sticky_buffer"]; ok {
 		val := parseExtraInt(v)
 		if val > 0 {
 			return val
 		}
 	}
+
 	base := a.GetBaseRPM()
-	buffer := base / 5
-	if buffer < 1 && base > 0 {
-		buffer = 1
+	if base <= 0 {
+		return 0
+	}
+
+	// Cache-driven buffer = concurrency + maxSessions
+	conc := a.Concurrency
+	if conc < 0 {
+		conc = 0
+	}
+	sess := a.GetMaxSessions()
+	if sess < 0 {
+		sess = 0
 	}
+
+	buffer := conc + sess
+
+	// floor: 向后兼容
+	floor := base / 5
+	if floor < 1 {
+		floor = 1
+	}
+	if buffer < floor {
+		buffer = floor
+	}
+
 	return buffer
 }
 

backend/internal/service/account_rpm_test.go

@@ -90,28 +90,47 @@ func TestCheckRPMSchedulability(t *testing.T) {
 
 func TestGetRPMStickyBuffer(t *testing.T) {
 	tests := []struct {
-		name     string
-		extra    map[string]any
-		expected int
+		name        string
+		concurrency int
+		extra       map[string]any
+		expected    int
 	}{
-		{"nil extra", nil, 0},
-		{"no keys", map[string]any{}, 0},
-		{"base_rpm=0", map[string]any{"base_rpm": 0}, 0},
-		{"base_rpm=1 min buffer 1", map[string]any{"base_rpm": 1}, 1},
-		{"base_rpm=4 min buffer 1", map[string]any{"base_rpm": 4}, 1},
-		{"base_rpm=5 buffer 1", map[string]any{"base_rpm": 5}, 1},
-		{"base_rpm=10 buffer 2", map[string]any{"base_rpm": 10}, 2},
-		{"base_rpm=15 buffer 3", map[string]any{"base_rpm": 15}, 3},
-		{"base_rpm=100 buffer 20", map[string]any{"base_rpm": 100}, 20},
-		{"custom buffer=5", map[string]any{"base_rpm": 10, "rpm_sticky_buffer": 5}, 5},
-		{"custom buffer=0 fallback to default", map[string]any{"base_rpm": 10, "rpm_sticky_buffer": 0}, 2},
-		{"custom buffer negative fallback", map[string]any{"base_rpm": 10, "rpm_sticky_buffer": -1}, 2},
-		{"custom buffer with float", map[string]any{"base_rpm": 10, "rpm_sticky_buffer": float64(7)}, 7},
-		{"json.Number base_rpm", map[string]any{"base_rpm": json.Number("10")}, 2},
+		// 基础退化
+		{"nil extra", 0, nil, 0},
+		{"no keys", 0, map[string]any{}, 0},
+		{"base_rpm=0", 0, map[string]any{"base_rpm": 0}, 0},
+
+		// 新公式: concurrency + maxSessions, floor = base/5
+		{"conc=3 sess=10 → 13", 3, map[string]any{"base_rpm": 15, "max_sessions": 10}, 13},
+		{"conc=2 sess=5 → 7", 2, map[string]any{"base_rpm": 10, "max_sessions": 5}, 7},
+		{"conc=3 sess=15 → 18", 3, map[string]any{"base_rpm": 30, "max_sessions": 15}, 18},
+
+		// floor 生效 (conc+sess < base/5)
+		{"conc=0 sess=0 base=15 → floor 3", 0, map[string]any{"base_rpm": 15}, 3},
+		{"conc=0 sess=0 base=10 → floor 2", 0, map[string]any{"base_rpm": 10}, 2},
+		{"conc=0 sess=0 base=1 → floor 1", 0, map[string]any{"base_rpm": 1}, 1},
+		{"conc=0 sess=0 base=4 → floor 1", 0, map[string]any{"base_rpm": 4}, 1},
+		{"conc=1 sess=0 base=15 → floor 3", 1, map[string]any{"base_rpm": 15}, 3},
+
+		// 手动 override
+		{"custom buffer=5", 3, map[string]any{"base_rpm": 10, "rpm_sticky_buffer": 5, "max_sessions": 10}, 5},
+		{"custom buffer=0 fallback", 3, map[string]any{"base_rpm": 10, "rpm_sticky_buffer": 0, "max_sessions": 10}, 13},
+		{"custom buffer negative fallback", 3, map[string]any{"base_rpm": 10, "rpm_sticky_buffer": -1, "max_sessions": 10}, 13},
+		{"custom buffer with float", 3, map[string]any{"base_rpm": 10, "rpm_sticky_buffer": float64(7)}, 7},
+
+		// 负值 clamp
+		{"negative concurrency clamped", -5, map[string]any{"base_rpm": 15, "max_sessions": 10}, 10},
+		{"negative maxSessions clamped", 3, map[string]any{"base_rpm": 15, "max_sessions": -5}, 3},
+
+		// 高并发低会话
+		{"conc=10 sess=5 → 15", 10, map[string]any{"base_rpm": 10, "max_sessions": 5}, 15},
+
+		// json.Number
+		{"json.Number base_rpm", 3, map[string]any{"base_rpm": json.Number("10"), "max_sessions": json.Number("5")}, 8},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			a := &Account{Extra: tt.extra}
+			a := &Account{Concurrency: tt.concurrency, Extra: tt.extra}
 			if got := a.GetRPMStickyBuffer(); got != tt.expected {
 				t.Errorf("GetRPMStickyBuffer() = %d, want %d", got, tt.expected)
 			}

backend/internal/service/gateway_service.go

@@ -1418,19 +1418,24 @@ func (s *GatewayService) SelectAccountWithLoadAwareness(ctx context.Context, gro
 				if containsInt64(routingAccountIDs, stickyAccountID) && !isExcluded(stickyAccountID) {
 					// 粘性账号在路由列表中，优先使用
 					if stickyAccount, ok := accountByID[stickyAccountID]; ok {
-						if s.isAccountSchedulableForSelection(stickyAccount) &&
+						var stickyCacheMissReason string
+
+						gatePass := s.isAccountSchedulableForSelection(stickyAccount) &&
 							s.isAccountAllowedForPlatform(stickyAccount, platform, useMixed) &&
 							(requestedModel == "" || s.isModelSupportedByAccountWithContext(ctx, stickyAccount, requestedModel)) &&
 							s.isAccountSchedulableForModelSelection(ctx, stickyAccount, requestedModel) &&
 							s.isAccountSchedulableForQuota(stickyAccount) &&
-							s.isAccountSchedulableForWindowCost(ctx, stickyAccount, true) &&
+							s.isAccountSchedulableForWindowCost(ctx, stickyAccount, true)
+
+						rpmPass := gatePass && s.isAccountSchedulableForRPM(ctx, stickyAccount, true)
 
-							s.isAccountSchedulableForRPM(ctx, stickyAccount, true) { // 粘性会话窗口费用+RPM 检查
+						if rpmPass { // 粘性会话窗口费用+RPM 检查
 							result, err := s.tryAcquireAccountSlot(ctx, stickyAccountID, stickyAccount.Concurrency)
 							if err == nil && result.Acquired {
 								// 会话数量限制检查
 								if !s.checkAndRegisterSession(ctx, stickyAccount, sessionHash) {
 									result.ReleaseFunc() // 释放槽位
+									stickyCacheMissReason = "session_limit"
 									// 继续到负载感知选择
 								} else {
 									if s.debugModelRoutingEnabled() {
@@ -1444,27 +1449,49 @@ func (s *GatewayService) SelectAccountWithLoadAwareness(ctx context.Context, gro
 								}
 							}
 
-							waitingCount, _ := s.concurrencyService.GetAccountWaitingCount(ctx, stickyAccountID)
-							if waitingCount < cfg.StickySessionMaxWaiting {
-								// 会话数量限制检查（等待计划也需要占用会话配额）
-								if !s.checkAndRegisterSession(ctx, stickyAccount, sessionHash) {
-									// 会话限制已满，继续到负载感知选择
+							if stickyCacheMissReason == "" {
+								waitingCount, _ := s.concurrencyService.GetAccountWaitingCount(ctx, stickyAccountID)
+								if waitingCount < cfg.StickySessionMaxWaiting {
+									// 会话数量限制检查（等待计划也需要占用会话配额）
+									if !s.checkAndRegisterSession(ctx, stickyAccount, sessionHash) {
+										stickyCacheMissReason = "session_limit"
+										// 会话限制已满，继续到负载感知选择
+									} else {
+										return &AccountSelectionResult{
+											Account: stickyAccount,
+											WaitPlan: &AccountWaitPlan{
+												AccountID:      stickyAccountID,
+												MaxConcurrency: stickyAccount.Concurrency,
+												Timeout:        cfg.StickySessionWaitTimeout,
+												MaxWaiting:     cfg.StickySessionMaxWaiting,
+											},
+										}, nil
+									}
 								} else {
-									return &AccountSelectionResult{
-										Account: stickyAccount,
-										WaitPlan: &AccountWaitPlan{
-											AccountID:      stickyAccountID,
-											MaxConcurrency: stickyAccount.Concurrency,
-											Timeout:        cfg.StickySessionWaitTimeout,
-											MaxWaiting:     cfg.StickySessionMaxWaiting,
-										},
-									}, nil
+									stickyCacheMissReason = "wait_queue_full"
 								}
 							}
 							// 粘性账号槽位满且等待队列已满，继续使用负载感知选择
+						} else if !gatePass {
+							stickyCacheMissReason = "gate_check"
+						} else {
+							stickyCacheMissReason = "rpm_red"
+						}
+
+						// 记录粘性缓存未命中的结构化日志
+						if stickyCacheMissReason != "" {
+							baseRPM := stickyAccount.GetBaseRPM()
+							var currentRPM int
+							if count, ok := rpmFromPrefetchContext(ctx, stickyAccount.ID); ok {
+								currentRPM = count
+							}
+							logger.LegacyPrintf("service.gateway", "[StickyCacheMiss] reason=%s account_id=%d session=%s current_rpm=%d base_rpm=%d",
+								stickyCacheMissReason, stickyAccountID, shortSessionHash(sessionHash), currentRPM, baseRPM)
 						}
 					} else {
 						_ = s.cache.DeleteSessionAccountID(ctx, derefGroupID(groupID), sessionHash)
+						logger.LegacyPrintf("service.gateway", "[StickyCacheMiss] reason=account_cleared account_id=%d session=%s current_rpm=0 base_rpm=0",
+							stickyAccountID, shortSessionHash(sessionHash))
 					}
 				}
 			}