Re: Move PinBuffer and UnpinBuffer to atomics

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

> 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? Because now, without timeout,
exclusive locker wouldn't be waked up until all shared locks are released.

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company