问题
So I have a situation where I may have many, many reads and only the occasional write to a resource shared between multiple threads.
A long time ago I read about ReaderWriterLock, and have read about ReaderWriterGate which attempts to mitigate the issue where many writes coming in trump reads and hurt performance. However, now I've become aware of ReaderWriterLockSlim...
From the docs, I believe that there can only be one thread in "upgradeable mode" at any one time. In a situation where the only access I'm using is EnterUpgradeableReadLock() (which is appropriate for my scenario) then is there much difference to just sticking with lock(){}?
Here's the excerpt:
A thread that tries to enter upgradeable mode blocks if there is already a thread in upgradeable mode, if there are threads waiting to enter write mode, or if there is a single thread in write mode.
Or, does the recursion policy make any difference to this?
回答1:
Agreed. If all of your threads need to acquire an upgradable read lock and you cannot afford to release a read lock and acquire a write lock then ReaderWriterLockSlim is no improvement over a simple exclusive lock. Recursion does not change that. RWLS and the need to avoid the ever present danger of deadlock heavily favors a pattern where a single thread does the writing.
回答2:
I don't have all your answers, but I'll give it a shot:
The lock statement in c# is syntactic sugar for calling Monitor.Enter and Monitor.Exit. The effect is that only one thread can access the code within the lock at a time.
lock()
{
//only one thread can access this code at a time
}
The issue with this is that multiple reads are harmless, but lock() blocks anyway. ReaderWriterLockSlim allows multiple reads, only one write. It's an attempt to improve efficiency.
The recursion policy is something you must specify - by default it is off. Don't know too much more beyond that, but hope that helps a little.
来源：https://stackoverflow.com/questions/2745352/is-readerwriterlockslim-enterupgradeablereadlock-essentially-the-same-as-monit