About Distributed Reader-Writer Mutex by Dmitry Vyukov

[aminer10]

Hello,

 Dmitry Vyukov  wrote this:

"In order to a reader-writer mutex to scale reader's time under mutex must

be significant, on the order of, let's say, 10'000 cycles. And the more hardware

threads you have the longer the time must be. Otherwise, inter-reader synchronization

overheads dominate, and the system does not scale."

Read here:

http://www.1024cores.net/home/lock-free-algorithms/reader-writer-problem/distributed-reader-writer-mutex

I have done some tests with my light weight MREW and my lockfree mpmc

fifo queue and what i think is that the CAS is generating a lot of contention this is is why

my rwlock is not scaling.

What you are doing Dmitry Vyukovin in your distributed rwlock is lowering the contention

using the same method as lock striping it is why it is scaling, but there is still a possibility

of contention that can cause a problem to the scalability if you are not setting the number

of rwlocks inside your distributed rwlock correctly.

other than that:

I have looked at the Distributed Reader-Writer Mutex by Dmitry Vyukov, look
at it here:"

http://www.1024cores.net/home/lock-free-algorithms/reader-writer-problem/distributed-reader-writer-mutex


and i think there is a problem with this method,  cause look at the write
lock function:


int         distr_rw_mutex_wrlock(distr_rw_mutex_t* mtx)
{
    int                     i;
    for (i = 0; i != mtx->proc_count; i += 1)
        pthread_rwlock_wrlock(&mtx->cell.mtx);
    return 0;
}


What is wrong with it ? suppose two or more writers want to lock this
distributed rwlock there is a possibility of a deadlock.

So i think you have to use a critical section around the for loop to be able
to lock all the rwlocks atomically to avoid the deadlock problem..




Thank you,
Amine Moulay Ramdane.