Subj : Concurrent reads and sequential writes
To   : comp.programming.threads
From : kozlovsky
Date : Tue Jul 26 2005 06:03 pm

Hello,

I am writing a spider application on Linux, C++, STL, pthreads.
The application has a cache that contains (word, handle) pairs.
The Cache class has method "get", that, given a word, returns its
handle (if found in the cache), and method "add", that adds a
new pair to the cache. About 95% of cache access are get's.

At present, the cache is synchronized with a single mutex, so
only a single thread can access the cache at any given time.
I would like to change synchronization in such a way that any
number of threads can call "get" at the same time, while "add"
must remain exclusive, i.e. only one thread can execute "add"
after blocking threads from starting new "get" and waiting
till threads currently executing "get" are all finished.

I cannot figure out how to do it with mutexes, although this
seems to be a quite common situation, is it? Should I use
semaphores? Or may be sig_atomic_t? Why should I try to
reinvent the bycicle? Perhaps there is a known simple
solution to my problem. Please, help.

     Gregory

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