Re: Some problems of recovery conflict wait events - Mailing list pgsql-hackers

On 2020/03/04 14:31, Masahiko Sawada wrote:
> On Wed, 4 Mar 2020 at 13:48, Fujii Masao <masao.fujii@oss.nttdata.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.

I started reading v2-0002-Improve-wait-events-for-recovery-conflict-resolut.patch.

-    ProcWaitForSignal(PG_WAIT_BUFFER_PIN);
+    ProcWaitForSignal(WAIT_EVENT_RECOVERY_CONFLICT_BUFFER_PIN);

Currently the wait event indicating the wait for buffer pin has already
been reported. But the above change in the patch changes the name of
wait event for buffer pin only in the startup process. Is this really useful?
Isn't the existing wait event for buffer pin enough?

-    /* Wait to be signaled by the release of the Relation Lock */
-    ProcWaitForSignal(PG_WAIT_LOCK | locktag.locktag_type);
+        /* Wait to be signaled by the release of the Relation Lock */
+        ProcWaitForSignal(WAIT_EVENT_RECOVERY_CONFLICT_LOCK);

Same as above. Isn't the existing wait event enough?

-    /*
-     * Progressively increase the sleep times, but not to more than 1s, since
-     * pg_usleep isn't interruptible on some platforms.
-     */
-    standbyWait_us *= 2;
-    if (standbyWait_us > 1000000)
-        standbyWait_us = 1000000;
+    WaitLatch(MyLatch,
+              WL_LATCH_SET | WL_POSTMASTER_DEATH | WL_TIMEOUT,
+              STANDBY_WAIT_MS,
+              wait_event_info);
+    ResetLatch(MyLatch);

ResetLatch() should be called before WaitLatch()?

Could you tell me why you dropped the "increase-sleep-times" logic?

Regards,

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