Christian Froeschlin <chr### [at] chrfrde> wrote:
> I don't know the pthread library well, but are you sure its
> lock function works by polling? I'd have expected it to wait
> for a signal sent by the unlock function and Thread 1 to
> get awakened before Thread 2 can lock again.

  You still need a list of threads waiting for the lock to become available.
You might have several threads waiting in line for the lock, and you can't
send a signal to all of them. You should send the signal to the one which
has been waiting the longest. IOW, you need a FIFO.

> Also, why should Thread 2 have a higher priority anyway? It
> may feel unfair for it to have to wait all the time, but if
> both threads lock the resource too frequently one of them
> is bound to be waiting all the time, so the throughput
> is not helped by fairness.

  Because if you don't distribute the lock evenly, you may get uneven load,
and the benefit of multithreading will suffer.

  For example, suppose you have a memory pool from which threads can
allocate and deallocate objects for whatever task they are performing.
If one thread is constantly obstructing another thread from allocating
objects from the pool (as might in theory happen in the way I described),
you will effectively have a single-threaded program rather than a
two-threaded one, because one of the threads will be idle (or seriously
underutilized) because its access to the memory pool is constantly being
blocked by the other thread.

  If the second thread happens to get access to the memory pool by chance,
the situation may reverse itself (iow. now it's the first thread who can't
get to the memory pool), but the end result is the same.

  (Granted, in practice it would be quite unlikely for a situation like
this to happen even with a naive mutex, especially if the threads
allocate and deallocate objects from the memory pool relatively
infrequently. However, the more frequently they do it, the more likely
it is that they will start obstructing each other, decreasing the overall
performance of the program.)

-- 
                                                          - Warp

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