Subject: Re: adding wait_start column to pg_locks
From: Fujii Masao <masao.fujii@oss.nttdata.com>
To: torikoshia <torikoshia@oss.nttdata.com>
Cc: Ian Lawrence Barwick <barwick@gmail.com>,
 Robert Haas <robertmhaas@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>,
 pgsql-hackers <pgsql-hackers@postgresql.org>
References: <a96013dc51cdc56b2a2b84fa8a16a993@oss.nttdata.com>
 <23d39ee9c31643fad8f9ba9c5cf3aaf4@oss.nttdata.com>
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 <7b1bb07a-73ec-1fcc-2f65-41101adf0080@oss.nttdata.com>
 <a5bfd1c2f2fb21eb7ea5ddec2e094430@oss.nttdata.com>
 <8002219d-b999-12fa-2327-49afe40e5fbb@oss.nttdata.com>
 <88c451c7-2729-4329-fbae-6ed7664adea1@oss.nttdata.com>
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 <3db333a9-31f7-a225-8038-eeb835ba8f37@oss.nttdata.com>
 <b8425207811a6b99d0ecd76844906a8e@oss.nttdata.com>
 <f77120a3-4762-bf71-57d5-f0be081715f7@oss.nttdata.com>
 <d45e44d655f2ba06d9e722d3dbcfdd79@oss.nttdata.com>
 <990e5d4b-075f-101f-aec5-bb44f9b30550@oss.nttdata.com>
 <40dfaa75-1058-e811-1f7c-4cf7203a3068@oss.nttdata.com>
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 <337d5311-6113-15e0-bd85-90b47b55f5ae@oss.nttdata.com>
Message-ID: <655f1a5b-4fa8-53ba-c3e2-8cee31587d37@oss.nttdata.com>
Date: Mon, 15 Feb 2021 15:17:51 +0900
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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!

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION