问题
We are developing a network application based C/S, we find there are too many locks adding to std::map that the performance of server became poor.
I wonder if it is possible to implement a lock-free map, if yes, how? Is there any open source code there?
EDIT: Actually we use the std::map to store sockets information, we did encapsulation based on the socket file description to include some other necessary information such as ip address, port, socket type, tcp or udp, etc.
To summary, we have a global map say it's
map<int fileDescriptor, socketInfor*> SocketsMap,
then every thread which is used to send data needs to access SocketsMap, and they have to add mutex before reading from SocketsMap or writing to SocketsMap, thus the concurrency level of the whole application would be greatly decreased because of so many locks addding to SocketsMap.
To avoid the concurrency level problem, we have two solutions: 1. store each socketInfor* separately 2. use some kind of lock-free map.
I would like to find some kind of lock-free map, because codes changes required by this solution are much less than that of solution 1.
回答1:
Actually there's a way, although I haven't implemented it myself there's a paper on a lock free map using hazard pointers from eminent C++ expert Andrei Alexandrescu.
回答2:
Yes, I have implemented a Lock-Free Unordered Map (docs) in C++ using the "Split-Ordered Lists" concept. It's an auto-expanding container and supports millions of elements on a 64-bit CAS without ABA issues. Performance-wise, it's a beast (see page 5). It's been extensively tested with millions of random ops.
回答3:
HashMap would suit? Have a look at Intel Threading Building Blocks, they have an interesting concurrent map. I'm not sure it's lock-free, but hopefully you're interested in good multithreading performance, not particularly in lock-freeness. Also you can check CityHash lib
EDIT:
Actually TBB's hash map is not lock-free
回答4:
If you use C++11, you can have a look at AtomicHashMap of facebook/folly
回答5:
You can implement the map using optimistic design or transactional memory.
This approach is especially effective if the chance of two operations concurrently addressing the map and one is changing the structure of it is relatively small - and you do not want the overhead of locking every time.
However, from time to time - collision will occur, and you will have to result it somehow (usually by rolling back to the last stable state and retrying the operations).
If your hardware support good enough atomic operations - this can be easily done with Compare And Swap (CAS) - where you change the reference alone (and whenever you change the map, you work on a copy of the map, and not the original, and set it as the primary only when you commit).
回答6:
I'm surprised nobody has mentioned it, but Click Cliff has implemented a wait-free hashmap in Java, which I believe could be ported to C++,
来源：https://stackoverflow.com/questions/14338732/is-it-possible-to-implement-lock-free-map-in-c