Writing in one, reading from another

[zennehoy]

Hi all,

what's the (realistic) worst case scenario that could happen if I read (iterate over equal_range) from a std::multimap in one thread while writing (inserting a new element) in another?

I have an application which will only very rarely write (few dozen times in the first seconds, maybe once every few minutes thereafter), but often read (every couple milliseconds), and I really don't want to have to check a mutex every millisecond for those rare writes.

My understanding of red-black trees (which I believe std::multimaps are) would lead me to believe that nothing horrible could happen, but then again my understanding of red-black trees is nowhere near perfect...

Any other ideas for avoiding a mutex? Or should I just mutex it and get it over with?

Thanks for the feedback!
Zen

[Codeplug]

Get it over with by doing it right.

The fast-path for acquiring a CRITICAL_SECTION is pretty darn fast. Since you rarely write, readers will take the fast-path most of the time.

gg

[ProgrammerC++]

What the hell is the fast-path of a criticalsection?

[Codeplug]

If there are no other threads trying to acquire the CS, then EnterCriticalSection can acquire the CS without making a kernel call. This is the "fast-path".

Otherwise, the thread has to make a kernel call in order to "block" until it can acquire the CS. This is the "slow-path".

InitializeCriticalSectionAndSpinCount() is a similar optimization in that it tries to avoid the kernel call.

gg

[ProgrammerC++]

If it doesnt need a kernel call, how does EnterCriticalSection make sure that function itself is thread safe?

I mean you could think EnterCriticalSection needs a critical section for itself.. to prevent two threads from going into the critical section at the same time.. lol

[Codeplug]

This is where the synchronization primitives provide by your OS or threading library "take care of business" They simply do what needs to be done for that particular OS/architecture.

Windows OS's run on x86, x64, IA64, MIPS, SHx, ARM, and PowerPC. What "needs to be done" can vary from one platform to the next.

gg