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From: Howard Hinnant (hinnant_at_[hidden])
Date: 2004-07-10 15:28:48

On Jul 9, 2004, at 11:00 PM, Glen Knowles wrote:

>> <nod> I just implemented lock_both::lock two ways: Once
>> just keep try-locking both ways, and once locking in the
>> order of the addresses of the locks (with only two locks in
>> the test, this was a safe operator<). My test looks like:
> The problem case for spinning is when one thread gets the locks and the
> holds them for a non-trivial length of time. During that time the
> other,
> spinning thread, just wastes cpu as fast as it can. This just gets
> magnified
> as more threads are involved.

You are describing a spin-lock, but not lock_both (except for a
theoretical and unlikely scenario).

On Jul 9, 2004, at 9:35 AM, Peter Dimov wrote:

> I don't have much experience with try locks, though. Pretty much the
> only
> example that comes to mind is Howard's lock_both:
> for(;;)
> {
> l1.lock();
> if( l2.try_lock() ) return;
> l1.unlock();
> l2.lock();
> if( l1.try_lock() ) return;
> l2.unlock();
> }
> (I hope I got that right) and I don't see how it can be improved by
> the if()
> idiom.

Yes Peter, you got it right.

If thread A already holds l1, thread B blocks, not spins, until thread
A unlocks l1. If l1 is initially unlocked, but thread A holds l2,
thread B locks l1, try and fails to lock l2, unlocks l1, then blocks
(not spins) on l2 until thread A unlocks l2.

It is possible for thread A to continually cause thread B to spin by
locking and unlocking l1 and l2 at just the right (or wrong!) times,
but in practice this dynamically unstable state does not appear to
persist. Specifically, in the case you site (thread A holding l1 and
l2 for a long time), thread B will simply block on l1 and not consume
cpu except the system time required for maintaining the block.

Implementing lock_both::lock with an ordering algorithm could, on the
other hand, more likely reduce throughput in comparison. Consider
three ordered locks: l1, l2, l3.

Thread A needs to lock l1 and l3, thread B needs to lock l2 and l3, and
thread C needs to lock l1:

Thread A Thread B Thread C
lock l1
                      lock l2
                      lock l3
block on l3
                                       block on l1

At this point thread B is causing both thread A and thread C to block.
However in the try-and-back-off formulation of lock_both the scenario
would be different:

Thread A Thread B Thread C
lock l1
                      lock l2
                      lock l3
try but fail l3
                                       block on l1
unlock l1
block on l3
                                       lock l1

Similar timing, but now both thread B and C are able to process
concurrently, only thread A is blocked. In a nutshell, if thread A
can't get both locks, it tries to put itself to sleep on whichever lock
it can't get in order to /not/ waste cpu. Additionally it holds no
locks while it is sleeping so as to increase potential throughput of
other threads.


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