SPMC | fork of xbmc/kodi - | Media Player library

I have a (Posix) server that acts as a proxy for many clients to another upstream server. Messages typically flow down from the upstream server, are then matched against, and pushed out to some subset of the clients interested in that traffic (maintaining the FIFO order from the upstream server). Currently, this proxy server is single threaded using an event loop (e.g. - select, epoll, etc.), but now I'd like to make it multithreaded so that the proxy can more fully utilize an entire machine and achieve much higher throughput.

My high level design is to have a pool of N worker pthreads (where N is some small multiple of the number of cores on the machine) who each run their own event loop. Each client connection will be assigned to a specific worker thread who would then be responsible for servicing all of that client's I/O + timeout needs for the duration of that client connection. I also intend to have a single dedicated thread who pulls in the messages in from the upstream server. Once a message is read in, its contents can be considered constant / unchanging, until it is no longer needed and reclaimed. The workers never alter the message contents -- they just pass them along to their clients as needed.

My first question is: should the matching of client interests preferably be done by the producer thread or the worker threads?

In the former approach, for each worker thread, the producer could check the interests (e.g. - group membership) of the worker's clients. If the message matched any clients, then it could push the message onto a dedicated queue for that worker. This approach requires some kind of synchronization between the producer and each worker about their client's rarely changing interests.

In the latter approach, the producer just pushes every message onto some kind of queue shared by all of the worker threads. Then each worker thread checks ALL of the messages for a match against their clients' interests and processes each message that matches. This is a twist on the usual SPMC problem where a consumer is usually assumed to unilaterally take an element for themselves, rather than all consumers needing to do some processing on every element. This approach distributes the matching work across multiple threads, which seems desirable, but I worry it may cause more contention between the threads depending on how we implement their synchronization.

In both approaches, when a message is no longer needed by any worker thread, it then needs to be reclaimed. So, some tracking needs to be done to know when no worker thread needs a message any longer.

My second question is: what is a good way of tracking whether a message is still needed by any of the worker threads?

A simple way to do this would be to assign to each message a count of how many worker threads still need to process the message when it is first produced. Then, when each worker is done processing a message it would decrement the count in a thread-safe manner and if/when the count went to zero we would know it could be reclaimed.

Another way to do this would be to assign 64b sequence numbers to the messages as they came in, then each thread could track and record the highest sequence number up through which they have processed somehow. Then we could reclaim all messages with sequence numbers less than or equal to the minimum processed sequence number across all of the worker threads in some manner.

The latter approach seems like it could more easily allow for a lazy reclamation process with less cross-thread synchronization necessary. That is, you could have a "clean-up" thread that only runs periodically who goes and computes the minimum across the worker threads, with much less inter-thread synchronization being necessary. For example, if we assume that reads and writes of a 64b integer are atomic and a worker's fully processed sequence number is always monotonically increasing, then the "clean-up" thread can just periodically read the workers' fully processed counts (maybe with some memory barrier) and compute the minimum.

Third question: what is the best way for workers to realize that they have new work to do in their queue(s)?

Each worker thread is going to be managing its own event loop of client file descriptors and timeouts. Is it best for each worker thread to just have their own pipe to which signal data can be written by the producer to poke them into action? Or should they just periodically check their queue(s) for new work? Are there better ways to do this?

Last question: what kind of data structure and synchronization should I use for the queue(s) between the producer and the consumer?

I'm aware of lock-free data structures but I don't have a good feel for whether they'd be preferable in my situation or if I should instead just go with a simple mutex for operations that affect the queue. Also, in the shared queue approach, I'm not entirely sure how a worker thread should track "where" it is in processing the queue.

Any insights would be greatly appreciated! Thanks!

I have a MyReader that implements Iterator and produces Buffers where Buffer : Send. MyReader produces a lot of Buffers very quickly, but I have a CPU-intensive job to perform on each Buffer (.map(|buf| ...)) that is my bottleneck, and then gather the results (ordered). I want to parallelize the CPU intense work - hopefully to N threads, that would use work stealing to perform them as fast as the number of cores allows.

Edit: To be more precise. I am working on rdedup. MyStruct is Chunker which reads io::Read (typically stdio), finds parts (chunks) of data and yields them. Then map() is supposed, for each chunk, to calculate sha256 digest of it, compress, encrypt, save and return the digest as the result of map(...). Digest of saved data is used to build index of the data. The order between chunks being processed by map(...) does not matter, but digest returned from each map(...) needs to be collected in the same order that the chunks were found. The actual save to file step is offloaded to yet another thread (writter thread). actual code of PR in question

I hoped I can use rayon for this, but rayon expect an iterator that is already parallizable - eg. a Vec<...> or something like that. I have found no way to get a par_iter from MyReader - my reader is very single-threaded in nature.

There is simple_parallel but documentation says it's not recommended for general use. And I want to make sure everything will just work.

I could just take a spmc queue implementation and a custom thread_pool, but I was hopping for an existing solution that is optimized and tested.

There's also pipeliner but doesn't support ordered map yet.