Re: [PATCH v9 0/5] Add NUMA-awareness to qspinlock

Alex Kogan
2020-01-27T16:02:52Z

Hi, Lihao.

>>>
>>>> This is particularly relevant
>>>> in high contention situations when new threads keep arriving on the same
>>>> socket as the lock holder.
>>> In this case, the lock will stay on the same NUMA node/socket for
>>> 2^numa_spinlock_threshold times, which is the worst case scenario if we
>>> consider the long-term fairness. And if we have multiple nodes, it will take
>>> up to 2^numa_spinlock_threshold X (nr_nodes - 1) + nr_cpus_per_node
>>> lock transitions until any given thread will acquire the lock
>>> (assuming 2^numa_spinlock_threshold > nr_cpus_per_node).
>>>
>>
>> You're right that the latest version of the patch handles long-term fairness
>> deterministically.
>>
>> As I understand it, the n-th thread in the main queue is guaranteed to
>> acquire the lock after N lock handovers, where N is bounded by
>>
>> n - 1 + 2^numa_spinlock_threshold * (nr_nodes - 1)
>>
>> I'm not sure what role the variable nr_cpus_per_node plays in your analysis.
>>
>> Do I miss anything?
>>
>
> If I understand correctly, there are two phases in the algorithm:
>
> MCS phase: when the secondary queue is empty, as explained in your emails,
> the algorithm hands the lock to threads in the main queue in an FIFO order.
> When probably(SHUFFLE_REDUCTION_PROB_ARG) returns false (with default
> probability 1%), if the algorithm finds the first thread running on the same
> socket as the lock holder in cna_scan_main_queue(), it enters the following
> CNA phase
Yep. When probably() returns false, we scan the main queue. If as the result of
this scan the secondary queue becomes not empty, we enter what you call
the CNA phase.

> .
>
> CNA phase: when the secondary queue is not empty, the algorithm keeps
> handing the lock to threads in the main queue that run on the same socket as
> the lock holder. When 2^numa_spinlock_threshold is reached, it splices
> the secondary queue to the front of the main queue. And we are back to the
> MCS phase above.
Correct.

> For the n-th thread T in the main queue, the MCS phase handles threads that
> arrived in the main queue before T. In high contention situations, the CNA
> phase handles two kinds of threads:
>
> 1. Threads ahead of T that run on the same socket as the lock holder when
> a transition from the MCS to CNA phase was made. Assume there are m such
> threads.
>
> 2. Threads that keep arriving on the same socket as the lock holder. There
> are at most 2^numa_spinlock_threshold of them.
>
> Then the number of lock handovers in the CNA phase is max(m,
> 2^numa_spinlock_threshold). So the total number of lock handovers before T
> acquires the lock is at most
>
> n - 1 + 2^numa_spinlock_threshold * (nr_nodes - 1)
>
> Please let me know if I misunderstand anything.
I think you got it right (modulo nr_cpus_per_node instead of n, as mentioned in
my other response).

Regards,
— Alex