Andriy Gapon wrote:
Brent, David,
thank you for the responses.
I think I incorrectly formulated my original concern.
It is not about behavior at mutex unlock but about behavior at mutex
re-lock. You are right that waking waiters at unlock would hurt
performance. But I think that it is not "fair" that at re-lock former
owner gets the lock immediately and the thread that waited on it for
longer time doesn't get a chance.
Here's a more realistic example than the mock up code.
Say you have a worker thread that processes queued requests and the load
is such that there is always something on the queue. Thus the worker
thread doesn't ever have to block waiting on it.
And let's say that there is a GUI thread that wants to convey some
information to the worker thread. And for that it needs to acquire some
mutex and "do something".
With current libthr behavior the GUI thread would never have a chance to
get the mutex as worker thread would always be a winner (as my small
program shows).
Or even more realistic: there should be a feeder thread that puts things
on the queue, it would never be able to enqueue new items until the
queue becomes empty if worker thread's code looks like the following:
while(1)
{
pthread_mutex_lock(&work_mutex);
while(queue.is_empty())
pthread_cond_wait(...);
//dequeue item
...
pthread_mutex_unlock(&work mutex);
//perform some short and non-blocking processing of the item
...
}
Because the worker thread (while the queue is not empty) would never
enter cond_wait and would always re-lock the mutex shortly after
unlocking it.
So while improving performance on small scale this mutex re-acquire-ing
unfairness may be hurting interactivity and thread concurrency and thus
performance in general. E.g. in the above example queue would always be
effectively of depth 1.
Something about "lock starvation" comes to mind.
So, yes, this is not about standards, this is about reasonable
expectations about thread concurrency behavior in a particular
implementation (libthr).
I see now that performance advantage of libthr over libkse came with a
price. I think that something like queued locks is needed. They would
clearly reduce raw throughput performance, so maybe that should be a new
(non-portable?) mutex attribute.
You forgot that default scheduling policy is time-sharing, after thread
#2 has blocked on the mutex for a while, when thread #1 unlocks the
mutex and unblocks thread #1, the thread #2's priority will be raised
and it preempts thread #1, the thread #2 then acquires the mutex,
that's how it balances between fairness and performance.
Regards,
David Xu
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