Re: Refactoring backend fork+exec code

On 23/01/2024 21:50, Andres Freund wrote:
> On 2024-01-23 21:07:08 +0200, Heikki Linnakangas wrote:
>> On 22/01/2024 23:07, Andres Freund wrote:
>>>> diff --git a/src/backend/utils/activity/backend_status.c b/src/backend/utils/activity/backend_status.c
>>>> index 1a1050c8da1..92f24db4e18 100644
>>>> --- a/src/backend/utils/activity/backend_status.c
>>>> +++ b/src/backend/utils/activity/backend_status.c
>>>> @@ -257,17 +257,16 @@ pgstat_beinit(void)
>>>> else
>>>> {
>>>> /* Must be an auxiliary process */
>>>> - Assert(MyAuxProcType != NotAnAuxProcess);
>>>> + Assert(IsAuxProcess(MyBackendType));
>>>> /*
>>>> * Assign the MyBEEntry for an auxiliary process. Since it doesn't
>>>> * have a BackendId, the slot is statically allocated based on the
>>>> - * auxiliary process type (MyAuxProcType). Backends use slots indexed
>>>> - * in the range from 0 to MaxBackends (exclusive), so we use
>>>> - * MaxBackends + AuxProcType as the index of the slot for an auxiliary
>>>> - * process.
>>>> + * auxiliary process type. Backends use slots indexed in the range
>>>> + * from 0 to MaxBackends (exclusive), and aux processes use the slots
>>>> + * after that.
>>>> */
>>>> - MyBEEntry = &BackendStatusArray[MaxBackends + MyAuxProcType];
>>>> + MyBEEntry = &BackendStatusArray[MaxBackends + MyBackendType - FIRST_AUX_PROC];
>>>> }
>>>
>>> Hm, this seems less than pretty. It's probably ok for now, but it seems like a
>>> better fix might be to just start assigning backend ids to aux procs or switch
>>> to indexing by pgprocno?
>>
>> Using pgprocno is a good idea. Come to think of it, why do we even have a
>> concept of backend ID that's separate from pgprocno? backend ID is used to
>> index the ProcState array, but AFAICS we could use pgprocno as the index to
>> that, too.
>
> I think we should do that. There are a few processes not participating in
> sinval, but it doesn't make enough of a difference to make sinval slower. And
> I think there'd be far bigger efficiency improvements to sinvaladt than not
> having a handful more entries.

And here we go. BackendID is now a 1-based index directly into the
PGPROC array.

--
Heikki Linnakangas
Neon (https://neon.tech)