Re: Notify enhancement

Tom Lane wrote:
> Alvaro Herrera <alvherre@commandprompt.com> writes:
>   
>> Andrew Dunstan wrote:
>>     
>>> I don't understand how we decide that everybody who needs a given 
>>> event+message has got it, if we don't know who (if anyone) is listening? 
>>> How do we decide that we no longer need the info in the shmem buffer? 
>>>       
>
>   
>> Keep a pointer in shared memory for each listener backend, saying how
>> far it has scanned the ring?  There would be a single writing pointer,
>> so it's trivial to see when the ring is "full".
>>     
>
> Right.  Read the code in src/backend/storage/ipc/sinvaladt.c, especially
> SIInsertDataEntry, SIGetDataEntry, SIDelExpiredDataEntries.
>
>   
>>> Are we keeping use of SIGUSR2 in this scheme?
>>>       
>
>   
>> What for?  Just protect the write pointer with a lwlock and have
>> listeners check whether somebody has written something.
>>     
>
> You do want something comparable to SIGUSR2 to prod active backends to
> consume messages, in case they are busy doing a query and hence not
> checking the ring.  I'm envisioning something like having the SIGUSR2
> signal handler set a flag that's checked by CHECK_FOR_INTERRUPTS(),
> and if set then ProcessInterrupts will go off and absorb messages.
> Onlookers can tell who's falling behind by noting where their read
> pointers are, and can issue SIGUSR2 to the laggards --- in particular,
> any backend that finds itself unable to insert a NOTIFY into the ring
> for lack of space can SIGUSR2 the laggards and then sleep a little.
>
>     
>   

OK, thanks. Will study further.

cheers

andrew