Abstract: Long-context Large Language Models (LLMs) have unlocked numerous possibilities for downstream applications, many of which involve multiple requests sharing the same input context. Recent inference frameworks like vLLM and SGLang, as well as LLMs providers such as OpenAI, Google and Anthropic, have employed prefix caching techniques to accelerate multi-requests with shared context. However, existing long-context methods are primarily evaluated on single query testing, failing to demonstrate their true capability in real-world applications that often require KV cache reuse for follow-up queries. To address this gap, we introduce SharedContextBench, a comprehensive long-context benchmark to reveal how lossy are long-context methods in KV cache reuse scenarios. Specifically, it encompasses 12 tasks with two shared context modes, covering four categories of long-context abilities: string retrieval, semantic retrieval, global information processing, and multi-task capabilities. Using our benchmark, we evaluated five categories of long-context solutions, including Gated Linear RNNs (Codestal-Mamba), Mamba-Attention hybrids (Jamba-1.5-Mini), and efficient methods like sparse attention, KV cache compression, and prompt compression, on six transformer-based long-context LLMs: Llama-3.1-8B/70B, Qwen2.5-72B/32B, Llama-3-8B-262K, and GLM-4-9B. Our findings show that sub-$O(n)$ memory methods often struggle to maintain accuracy in multi-turn scenarios, while sparse encoding methods with $O(n)$ memory and sub-$O(n^2)$ computation in prefilling generally perform well. Additionally, dynamic sparse patterns in prefilling often produce more expressive memory (KV cache) compared to static methods, and layer-level sparsity in hybrid architectures reduces memory usage while yielding promising results.