摘要： 海量小文件高并发访问场景下，元数据操作的频繁上下文切换构成用户态文件系统的主要性能瓶颈，现有缓存与预取机制难以在内存开销、随机访问适应性与性能之间取得有效平衡。针对用户态文件系统框架FUSE（Filesystem in Userspace）在多层级路径解析过程中的逐级查找开销问题，提出一种基于路径感知的批量元数据预取方案Lookupplus。 Lookupplus的核心设计思路是改变内核与用户态守护进程之间的请求交互粒度。在原生FUSE中，路径解析采用逐组件串行处理模式：每当某个组件的目录项缓存失效，FUSE内核模块即发起一次独立查找请求，每个失效组件均伴随两次上下文切换。随着目录层级加深，多次往返的累积开销显著放大元数据访问延迟。Lookupplus观察到，当某一中间组件缓存失效时，该路径上的剩余后续组件在极短时间内也面临重新验证需求。基于此，Lookupplus在首次失效组件触发查找请求时，从当前进程控制块中捕获完整剩余路径，附加在当前查找请求中，传递至用户态守护进程；守护进程解析剩余路径中的每个组件，返回其元数据条目，令内核批量更新各子目录项的属性和有效期，允许后续组件从快速路径验证。用户态文件系统无需感知批量预取逻辑或维护额外缓存一致性状态，新增内存开销仅来自临时路径拷贝和批量响应条目，随请求处理完毕即释放。 Lookupplus原型系统在Linux 5.2.2内核上实现，与现有先进方案ExtFUSE，RFUSE进行对比评估。单线程元数据性能测试表明，在目录深度为10时，Lookupplus的创建操作平均延迟降至原生FUSE的32%；在深度从1递增至10的连续运行过程中，查找请求总数由FUSE的65次降至20次。32线程微基准测试覆盖目录宽度从10到1,200的多种配置，预创建80%文件集。结果显示，Lookupplus对并发场景下的单一文件集访问性能提升低于5%。同时，ExtFUSE较原生FUSE低约20%；RFUSE则因队列压力在窄目录下吞吐量显著低于其他方案，随着目录宽度增加，逐步恢复至高吞吐水准。在典型真实应用宏基准测试中：文件服务器负载下Lookupplus吞吐量较原生FUSE提升7.4%；网络服务器负载下Lookupplus表现最优，较原生FUSE提升9.7%，较ExtFUSE提升12.9%；邮件服务器负载因目录结构扁平且同步开销显著，Lookupplus仅小幅领先。内存开销评估表明，Lookupplus在各负载下的Slab增量与原生FUSE基本一致，且绝大部分为内核可回收内存。 综上所述，Lookupplus通过路径级批量元数据预取原语，以最小架构扰动有效降低了FUSE在深层目录场景下的上下文切换开销。该方案不依赖复杂的缓存一致性协议和预测模型，不新增内核数据结构，额外内存开销可控且可回收，在深层目录、冷启动及低复用率场景下收益最为显著，为元数据密集型场景下构建高性能用户态文件系统提供了一条轻量可行的优化路径。

Abstract: In high-concurrency access scenarios with massive small files, frequent context switches caused by metadata operations become a major performance bottleneck for user-space file systems. Existing caching and prefetching mechanisms find it difficult to strike an effective balance among memory overhead, adaptability to random access patterns, and performance. To reduce the per-component lookup overhead during multi-level path resolution in the FUSE (Filesystem in Userspace) framework, this paper proposes a path-aware batch metadata prefetching scheme named Lookupplus. The core design philosophy of Lookupplus is to change the request interaction granularity between the kernel and the user-space daemon. In native FUSE, path resolution follows a serial per-component processing mode: whenever the directory entry cache of a component expires, the FUSE kernel module issues an independent lookup request, and each expired component incurs two context switches. As directory depth increases, the cumulative overhead of multiple round trips significantly amplifies metadata access latency. Lookupplus observes that when the cache of an intermediate component expires, the remaining subsequent components on that path will also require revalidation within a very short time. Based on this observation, when the first expired component triggers a lookup request, Lookupplus captures the complete remaining path from the current process control block, attaches it to the current lookup request, and transmits it to the user-space daemon. The daemon parses each component in the remaining path and returns their metadata entries, allowing the kernel to update the attributes and validity periods of all child directory entries in a batch, thereby enabling subsequent components to be verified through the fast path. The user-space file system does not need to be aware of the batch prefetching logic or maintain additional cache consistency states. The additional memory overhead comes only from temporary path copying and batch response entries, and is released immediately after request processing completes. The prototype system of Lookupplus is implemented on the Linux 5.2.2 kernel and evaluated against state-of-the-art schemes including ExtFUSE and RFUSE. Single-threaded metadata performance tests show that at a directory depth of 10, the average latency of create operations in Lookupplus drops to 32% of that in native FUSE. During continuous runs across depths from 1 to 10, the total number of lookup requests falls from 65 in native FUSE to 20 in Lookupplus. Thirty-two-thread micro-benchmarks cover directory widths ranging from 10 to 1,200, with 80% of the file set pre-created. Results show that Lookupplus achieves less than 5% performance improvement in concurrent access to a shared file set. Meanwhile, ExtFUSE performs approximately 20% lower than native FUSE. RFUSE shows significantly lower throughput under narrow directory configurations due to queue pressure, but gradually recovers to a high throughput level as directory width increases. In typical real-world macro-benchmarks, under the file server workload, Lookupplus improves throughput by 7.4% over native FUSE; under the web server workload, Lookupplus achieves the highest throughput, with a 9.7% improvement over native FUSE and a 12.9% improvement over ExtFUSE; under the mail server workload, due to the flat directory structure and significant synchronization overhead, Lookupplus achieves only a marginal improvement. Memory overhead evaluation shows that the Slab increment of Lookupplus under various workloads remains close to that of native FUSE, with the vast majority being kernel-reclaimable memory. In summary, Lookupplus effectively reduces the context-switch overhead of FUSE in deep-directory scenarios through a path-level batch metadata prefetching primitive with minimal architectural changes. It does not rely on complex cache consistency protocols or prediction models, nor does it introduce new kernel data structures. The extra memory overhead remains controllable and reclaimable. The benefits are most pronounced in deep-directory, cold-start, and low-reuse scenarios, making Lookupplus a lightweight and feasible optimization path for building high-performance user-space file systems in metadata-intensive environments.