Re: Move PinBuffer and UnpinBuffer to atomics

On Wed, Dec 9, 2015 at 2:17 PM, Alexander Korotkov <
a(dot)korotkov(at)postgrespro(dot)ru> wrote:

> On Tue, Dec 8, 2015 at 6:00 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> wrote:
>
>> On Tue, Dec 8, 2015 at 3:56 PM, Alexander Korotkov <
>> a(dot)korotkov(at)postgrespro(dot)ru> wrote:
>>>
>>> ​Agree. This patch need to be carefully verified. Current experiments
>>> just show that it is promising direction for improvement. I'll come with
>>> better version of this patch.
>>>
>>> Also, after testing on large machines I have another observation to
>>> share. For now, LWLock doesn't guarantee that exclusive lock would be ever
>>> acquired (assuming each shared lock duration is finite). It because when
>>> there is no exclusive lock, new shared locks aren't queued and LWLock state
>>> is changed directly. Thus, process which tries to acquire exclusive lock
>>> have to wait for gap in shared locks.
>>>
>>
>> I think this has the potential to starve exclusive lockers in worst case.
>>
>>
>>> But with high concurrency for shared lock that could happen very rare,
>>> say never.
>>>
>>> We did see this on big Intel machine in practice. pgbench -S gets shared
>>> ProcArrayLock very frequently. Since some number of connections is
>>> achieved, new connections hangs on getting exclusive ProcArrayLock. I think
>>> we could do some workaround for this problem. For instance, when exclusive
>>> lock waiter have some timeout it could set some special bit which prevents
>>> others to get new shared locks.
>>>
>>>
>> I think timeout based solution would lead to giving priority to
>> exclusive lock waiters (assume a case where each of exclusive
>> lock waiter timesout one after another) and make shared lockers
>> wait and a timer based solution might turn out to be costly for
>> general cases where wait is not so long.
>>
>
> ​Since all lwlock waiters are ordered in the queue, we can let only first
> waiter to set this bit.​
>

Thats okay, but still every time an Exclusive locker woke up, the
threshold time for its wait might be already over and it will set the
bit. In theory, that looks okay, but as compare to current algorithm
it will make more shared lockers to be added into wait queue.

> Anyway, once bit is set, shared lockers would be added to the queue. They
> would get the lock in queue order.
>
>

Ye thats right, but I think in general the solution to this problem
should be don't let any Exclusive locker to starve and still allow
as many shared lockers as possible. I think here it is important
how we define starving, should it be based on time or something
else? I find timer based solution somewhat less suitable, but may
be it is okay, if there is no other better way.

> Another way could be to
>> check if the Exclusive locker needs to go for repeated wait for a
>> couple of times, then we can set such a bit.
>>
>
> ​I'm not sure what do you mean by repeated wait. Do you mean exclusive
> locker was waked twice up by timeout?
>

I mean to say once the Exclusive locker is woken up, it again
re-tries to acquire the lock as it does today, but if it finds that the
number of retries is greater than certain threshold (let us say 10),
then we sit the bit.

> Because now, without timeout, exclusive locker wouldn't be waked up until
> all shared locks are released.
>
>
Does LWLockWakeup() work that way? I thought it works such
that once an Exclusive locker is encountered in the wait queue, it
just wakes that and won't try to wake any further waiters.

With Regards,
Amit Kapila.
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