.Net Mutex Question

Question

Hello,

I have an application that uses a Mutex for cross process synchronization of a block of code. This mechanism works great for the applications current needs. In the worst case I have noticed that about 6 threads can backup on the Mutex. It takes about 2-3 seconds to execute the synchronized code block.

I just received a new requirement that is asking to create a priority feature to the Mutex such that occasionally some requests of the Mutex can be deemed more important then the rest. When one of these higher priority threads comes in the desired functionality is for the Mutex to grant acquisition to the higher priority request instead of the lower.

So is there anyway to control the blocked Mutex queue that Windows maintains? Should I consider using a different threading model?

Thanks, Matt

Answer 1

Hi

Mutex isnt that great for a number of reasons, and as far as i know, there is no way to change promote one thread over another while they are running, nor a nice way to accomodate your requirement. I just read Jeffrey Richters "clr via c# 3", and there are a load of great thread sync constructs in there, and lots of good threading advice generally.

I wish i could remember enough of it to answer your question, but i doubt i would get it across as well as he can. check out his website: http://www.wintellect.com/ or search for some of his concurrent affairs articles. they will definitely help.

Answer 2

Using just the Mutex this will be tough one to solve, I am sure someone out there is thinking about thread priorities etc. but I would probably not consider this route.

One option would be to maintain a shared memory structure and implement a simple priority queue. The shared memory can use a MemoryMappedFile, then when a process wants to execute the section of code it puts a token with a priority on the priority queue and then when it wakes up each thread inspects the priority queue to check the first token in the queue if the token belongs to the process it can dequeue the token and execute the code.

Answer 3

Give each thread an AutoResetEvent. Then instead of waiting on a mutex, each thread adds its ARE to to a sorted list. If there is only one ARE on the list, fire the event, else wait for its ARE to fire. When a thread finishes processing, it removes its ARE from the list and fires the next one. Be sure to synchronize the list.