Re: Some problems of recovery conflict wait events

On 2020/03/05 20:16, Fujii Masao wrote:
>
>
> On 2020/03/05 16:58, Masahiko Sawada wrote:
>> On Wed, 4 Mar 2020 at 15:21, Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> wrote:
>>>
>>>
>>>
>>> On 2020/03/04 14:31, Masahiko Sawada wrote:
>>>> On Wed, 4 Mar 2020 at 13:48, Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> wrote:
>>>>>
>>>>>
>>>>>
>>>>> On 2020/03/04 13:27, Michael Paquier wrote:
>>>>>> On Wed, Mar 04, 2020 at 01:13:19PM +0900, Masahiko Sawada wrote:
>>>>>>> Yeah, so 0001 patch sets existing wait events to recovery conflict
>>>>>>> resolution. For instance, it sets (PG_WAIT_LOCK | LOCKTAG_TRANSACTION)
>>>>>>> to the recovery conflict on a snapshot. 0003 patch improves these wait
>>>>>>> events by adding the new type of wait event such as
>>>>>>> WAIT_EVENT_RECOVERY_CONFLICT_SNAPSHOT. Therefore 0001 (and 0002) patch
>>>>>>> is the fix for existing versions and 0003 patch is an improvement for
>>>>>>> only PG13. Did you mean even 0001 patch doesn't fit for back-patching?
>>>>>
>>>>> Yes, it looks like a improvement rather than bug fix.
>>>>>
>>>>
>>>> Okay, understand.
>>>>
>>>>>> I got my eyes on this patch set.  The full patch set is in my opinion
>>>>>> just a set of improvements, and not bug fixes, so I would refrain from
>>>>>> back-backpatching.
>>>>>
>>>>> I think that the issue (i.e., "waiting" is reported twice needlessly
>>>>> in PS display) that 0002 patch tries to fix is a bug. So it should be
>>>>> fixed even in the back branches.
>>>>
>>>> So we need only two patches: one fixes process title issue and another
>>>> improve wait event. I've attached updated patches.
>>>
>>> Thanks for updating the patches! I started reading 0001 patch.
>>
>> Thank you for reviewing this patch.
>>
>>>
>>> -                       /*
>>> -                        * Report via ps if we have been waiting for more than 500 msec
>>> -                        * (should that be configurable?)
>>> -                        */
>>> -                       if (update_process_title && new_status == NULL &&
>>> -                               TimestampDifferenceExceeds(waitStart, GetCurrentTimestamp(),
>>> -                                                                                  500))
>>>
>>> The patch changes ResolveRecoveryConflictWithSnapshot() and
>>> ResolveRecoveryConflictWithTablespace() so that they always add
>>> "waiting" into the PS display, whether wait is really necessary or not.
>>> But isn't it better to display "waiting" in PS basically when wait is
>>> necessary, like originally ResolveRecoveryConflictWithVirtualXIDs()
>>> does as the above?
>>
>> You're right. Will fix it.
>>
>>>
>>>    ResolveRecoveryConflictWithDatabase(Oid dbid)
>>>    {
>>> +       char            *new_status = NULL;
>>> +
>>> +       /* Report via ps we are waiting */
>>> +       new_status = set_process_title_waiting();
>>>
>>> In  ResolveRecoveryConflictWithDatabase(), there seems no need to
>>> display "waiting" in PS because no wait occurs when recovery conflict
>>> with database happens.
>>
>> Isn't the startup process waiting for other backend to terminate?
>
> Yeah, you're right. I agree that "waiting" should be reported in this case.

On second thought, in recovery conflict case, "waiting" should be reported
while waiting for the specified delay (e.g., by max_standby_streaming_delay)
until the conflict is resolved. So IMO reporting "waiting" in the case of
recovery conflict with buffer pin, snapshot, lock and tablespace seems valid,
because they are user-visible "expected" wait time.

However, in the case of recovery conflict with database, the recovery
basically doesn't wait at all and just terminates the conflicting sessions
immediately. Then the recovery waits for all those sessions to be terminated,
but that wait time is basically small and should not be the user-visible.
If that wait time becomes very long because of unresponsive backend, ISTM
that LOG or WARNING should be logged instead of reporting something in
PS display. I'm not sure if that logging is really necessary now, though.
Therefore, I'm thinking that "waiting" doesn't need to be reported in the case
of recovery conflict with database. Thought?

Regards,

--
Fujii Masao
NTT DATA CORPORATION
Advanced Platform Technology Group
Research and Development Headquarters