Optimizing std::unordered_map performance during testing phase in C++
I'm collaborating on a project where I'm testing a new approach and Can someone help me understand Hey everyone, I'm running into an issue that's driving me crazy... Currently developing a backend service that relies heavily on `std::unordered_map` for caching user sessions. In the testing phase, I've noticed significant performance drops when handling large datasets, particularly with 10,000+ entries. Profiling indicates that lookups are taking longer than expected, which seems counterintuitive given the average O(1) complexity. To tackle this, I've experimented with different hash functions by customizing `std::hash` for my user session keys. Here's a snippet of my current hash implementation: ```cpp struct SessionKey { std::string userId; std::string sessionId; bool operator==(const SessionKey& other) const { return userId == other.userId && sessionId == other.sessionId; } }; namespace std { template <> struct hash<SessionKey> { size_t operator()(const SessionKey& k) const { return hash<string>()(k.userId) ^ hash<string>()(k.sessionId); } }; } ``` I also tried reserving space in the unordered map upfront using `reserve()` to minimize reallocations. The initial capacity is set based on my expected load factor, but it hasn't made a noticeable difference. Here’s how I did that: ```cpp std::unordered_map<SessionKey, UserSession> sessionCache; sessionCache.reserve(10000); // anticipating 10,000 entries ``` Despite these tweaks, the user session retrieval times still lag behind what I consider acceptable. Moreover, I've been checking for any potential memory fragmentation issues using tools like Valgrind, but nothing alarming is reported. Another angle I've explored is switching to a `std::map` instead, as I’ve heard it might provide better performance with certain data distributions. However, I’d prefer to stay with `std::unordered_map` due to its average-case performance benefits. Has anyone else faced performance issues with `std::unordered_map` in similar scenarios? Are there alternative strategies or profiling techniques I might be overlooking? Any insights or advice would be greatly appreciated! I'm working on a web app that needs to handle this. My development environment is Debian. This issue appeared after updating to C++ latest. I'm open to any suggestions. Is there a simpler solution I'm overlooking? I'm developing on Windows 11 with C++. Could someone point me to the right documentation?