Re: adding wait_start column to pg_locks

On 2021-02-16 16:59, Fujii Masao wrote:
> On 2021/02/15 15:17, Fujii Masao wrote:
>>
>>
>> On 2021/02/10 10:43, Fujii Masao wrote:
>>>
>>>
>>> On 2021/02/09 23:31, torikoshia wrote:
>>>> On 2021-02-09 22:54, Fujii Masao wrote:
>>>>> On 2021/02/09 19:11, Fujii Masao wrote:
>>>>>>
>>>>>>
>>>>>> On 2021/02/09 18:13, Fujii Masao wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 2021/02/09 17:48, torikoshia wrote:
>>>>>>>> On 2021-02-05 18:49, Fujii Masao wrote:
>>>>>>>>> On 2021/02/05 0:03, torikoshia wrote:
>>>>>>>>>> On 2021-02-03 11:23, Fujii Masao wrote:
>>>>>>>>>>>> 64-bit fetches are not atomic on some platforms. So spinlock
>>>>>>>>>>>> is necessary when updating "waitStart" without holding the
>>>>>>>>>>>> partition lock? Also GetLockStatusData() needs spinlock when
>>>>>>>>>>>> reading "waitStart"?
>>>>>>>>>>>
>>>>>>>>>>> Also it might be worth thinking to use 64-bit atomic
>>>>>>>>>>> operations like
>>>>>>>>>>> pg_atomic_read_u64(), for that.
>>>>>>>>>>
>>>>>>>>>> Thanks for your suggestion and advice!
>>>>>>>>>>
>>>>>>>>>> In the attached patch I used pg_atomic_read_u64() and
>>>>>>>>>> pg_atomic_write_u64().
>>>>>>>>>>
>>>>>>>>>> waitStart is TimestampTz i.e., int64, but it seems
>>>>>>>>>> pg_atomic_read_xxx and pg_atomic_write_xxx only supports
>>>>>>>>>> unsigned int, so I cast the type.
>>>>>>>>>>
>>>>>>>>>> I may be using these functions not correctly, so if something
>>>>>>>>>> is wrong, I would appreciate any comments.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> About the documentation, since your suggestion seems better
>>>>>>>>>> than v6, I used it as is.
>>>>>>>>>
>>>>>>>>> Thanks for updating the patch!
>>>>>>>>>
>>>>>>>>> +    if (pg_atomic_read_u64(&MyProc->waitStart) == 0)
>>>>>>>>> +        pg_atomic_write_u64(&MyProc->waitStart,
>>>>>>>>> +                           
>>>>>>>>> pg_atomic_read_u64((pg_atomic_uint64 *) &now));
>>>>>>>>>
>>>>>>>>> pg_atomic_read_u64() is really necessary? I think that
>>>>>>>>> "pg_atomic_write_u64(&MyProc->waitStart, now)" is enough.
>>>>>>>>>
>>>>>>>>> +        deadlockStart =
>>>>>>>>> get_timeout_start_time(DEADLOCK_TIMEOUT);
>>>>>>>>> +        pg_atomic_write_u64(&MyProc->waitStart,
>>>>>>>>> +                    pg_atomic_read_u64((pg_atomic_uint64 *)
>>>>>>>>> &deadlockStart));
>>>>>>>>>
>>>>>>>>> Same as above.
>>>>>>>>>
>>>>>>>>> +        /*
>>>>>>>>> +         * Record waitStart reusing the deadlock timeout
>>>>>>>>> timer.
>>>>>>>>> +         *
>>>>>>>>> +         * It would be ideal this can be synchronously done
>>>>>>>>> with updating
>>>>>>>>> +         * lock information. Howerver, since it gives
>>>>>>>>> performance impacts
>>>>>>>>> +         * to hold partitionLock longer time, we do it here
>>>>>>>>> asynchronously.
>>>>>>>>> +         */
>>>>>>>>>
>>>>>>>>> IMO it's better to comment why we reuse the deadlock timeout
>>>>>>>>> timer.
>>>>>>>>>
>>>>>>>>>      proc->waitStatus = waitStatus;
>>>>>>>>> +    pg_atomic_init_u64(&MyProc->waitStart, 0);
>>>>>>>>>
>>>>>>>>> pg_atomic_write_u64() should be used instead? Because waitStart
>>>>>>>>> can be
>>>>>>>>> accessed concurrently there.
>>>>>>>>>
>>>>>>>>> I updated the patch and addressed the above review comments.
>>>>>>>>> Patch attached.
>>>>>>>>> Barring any objection, I will commit this version.
>>>>>>>>
>>>>>>>> Thanks for modifying the patch!
>>>>>>>> I agree with your comments.
>>>>>>>>
>>>>>>>> BTW, I ran pgbench several times before and after applying
>>>>>>>> this patch.
>>>>>>>>
>>>>>>>> The environment is virtual machine(CentOS 8), so this is
>>>>>>>> just for reference, but there were no significant difference
>>>>>>>> in latency or tps(both are below 1%).
>>>>>>>
>>>>>>> Thanks for the test! I pushed the patch.
>>>>>>
>>>>>> But I reverted the patch because buildfarm members rorqual and
>>>>>> prion don't like the patch. I'm trying to investigate the cause
>>>>>> of this failures.
>>>>>>
>>>>>> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=rorqual&dt=2021-02-09%2009%3A20%3A10
>>>>>
>>>>> -    relation     | locktype |        mode
>>>>> ------------------+----------+---------------------
>>>>> - test_prepared_1 | relation | RowExclusiveLock
>>>>> - test_prepared_1 | relation | AccessExclusiveLock
>>>>> -(2 rows)
>>>>> -
>>>>> +ERROR:  invalid spinlock number: 0
>>>>>
>>>>> "rorqual" reported that the above error happened in the server
>>>>> built with
>>>>> --disable-atomics --disable-spinlocks when reading pg_locks after
>>>>> the transaction was prepared. The cause of this issue is that
>>>>> "waitStart"
>>>>> atomic variable in the dummy proc created at the end of prepare
>>>>> transaction
>>>>> was not initialized. I updated the patch so that
>>>>> pg_atomic_init_u64() is
>>>>> called for the "waitStart" in the dummy proc for prepared
>>>>> transaction.
>>>>> Patch attached. I confirmed that the patched server built with
>>>>> --disable-atomics --disable-spinlocks passed all the regression
>>>>> tests.
>>>>
>>>> Thanks for fixing the bug, I also tested v9.patch configured with
>>>> --disable-atomics --disable-spinlocks on my environment and
>>>> confirmed
>>>> that all tests have passed.
>>>
>>> Thanks for the test!
>>>
>>> I found another bug in the patch. InitProcess() initializes
>>> "waitStart",
>>> but previously InitAuxiliaryProcess() did not. This could cause
>>> "invalid
>>> spinlock number" error when reading pg_locks in the standby server.
>>> I fixed that. Attached is the updated version of the patch.
>>
>> I pushed this version. Thanks!
>
> While reading the patch again, I found two minor things.
>
> 1. As discussed in another thread [1], the atomic variable "waitStart"
> should
> be initialized at the postmaster startup rather than the startup of
> each
> child process. I changed "waitStart" so that it's initialized in
> InitProcGlobal() and also reset to 0 by using pg_atomic_write_u64()
> in
> InitProcess() and InitAuxiliaryProcess().
>
> 2. Thanks to the above change, InitProcGlobal() initializes "waitStart"
> even in PGPROC entries for prepare transactions. But those entries
> are
> zeroed in MarkAsPreparingGuts(), so "waitStart" needs to be
> initialized
> again. Currently TwoPhaseGetDummyProc() initializes "waitStart" in
> the
> PGPROC entry for prepare transaction. But it's better to do that in
> MarkAsPreparingGuts() instead because that function initializes other
> PGPROC variables. So I moved that initialization code from
> TwoPhaseGetDummyProc() to MarkAsPreparingGuts().
>
> Patch attached. Thought?

Thanks for updating the patch!

It seems to me that the modification is right.
I ran some regression tests but didn't find problems.

Regards,

--
Atsushi Torikoshi