On Wednesday, 6 November 2019 16:43:52 GMT Waqar Khan wrote:
Hi Barry, Thanks for the response. Where can I read more about (1). It seems like that is something I need to explore. As we already have (2) (cache for each process). Thanks again for your help.
We use the UDS (Unix domain sockets) to talk to a master process. Twisted has support for this. But you need a small patch to avoid data lose. UDS does not lose data and is message based, not bytes based. We use pickle to encode requests and responses. Barry The patch is: --- Twisted-18.4.0.orig/src/twisted/internet/unix.py.orig 2018-08-01 12:45:38.711115425 +0100 +++ Twisted-18.4.0/src/twisted/internet/unix.py 2018-08-01 12:45:47.946115123 +0100 @@ -509,11 +509,6 @@ return self.write(datagram, address) elif no == EMSGSIZE: raise error.MessageLengthError("message too long") - elif no == EAGAIN: - # oh, well, drop the data. The only difference from UDP - # is that UDP won't ever notice. - # TODO: add TCP-like buffering - pass else: raise You then have to handle the EAGAIN error and do retries yourself. As it stands the patch is not good enough to put into twisted as a full fix would need to put the handling of the retries into twisted. I guess (2) does not work for you as the cache hit rate is low and you need to share the cache to get a benefit. Cache entries only get used a few times? In our case the hit rate is high (99%+) and we just pay the cost of populating the caches on process start up, which ends up being noise. Barry
On Wed, Nov 6, 2019 at 8:39 AM Scott, Barry <barry.scott@forcepoint.com>
wrote:
On Wednesday, 6 November 2019 14:21:22 GMT Maarten ter Huurne wrote:
On Wednesday, 6 November 2019 07:19:56 CET Waqar Khan wrote:
Hi, So, I am writing a twisted server. This server spawn multiple child processes using reactor spawnProcess that initializes a process protocol.
Now, each of the childprocess receives some REST requests. Each process has a dict that acts as cache. Now, I want to share dict across processes. In general, python has SharedMemoryManager in multiprocessing module which would have helped. https://docs.python.org/3/library/multiprocessing.shared_memory.html#m ultiprocessing.managers.SharedMemoryManager.SharedMemory But since I am using twisted internal process implementation, how do I share this dict across the processes so that all the processes use this common cache?
Keeping a dictionary in SharedMemoryManager seems far from trivial. I don't think you can allocate arbitrary Python objects in the shared memory and even if you could, you would run into problems when one process mutates the dictionary while another is looking up something or also mutating it.
It could in theory work if you implement a custom lock-less dictionary, but that would be a lot of work and hard to get right. Also having shared memory mutations be synced between multiple CPU cores could degrade performance, since keeping core-local CPU caches in sync is expensive.
Would it be an option to have only one process accept the REST requests, check whether the result is in the cache and only distribute work to the other processes if you get a cache miss? Typically the case where an answer is cached is pretty fast, so perhaps you don't need multiple processes to handle incoming requests.
We have used a couple of ways to cache. 1. Use a singleton process to hold the cache and ask it, via IPC, for answers from the other process. 2. have a cache in each process
Barry
Bye,
Maarten
_______________________________________________ Twisted-Python mailing list Twisted-Python@twistedmatrix.com https://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
_______________________________________________ Twisted-Python mailing list Twisted-Python@twistedmatrix.com https://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python