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Date: Thu, 18 Feb 2021 16:26:58 +0900
From: torikoshia <torikoshia@oss.nttdata.com>
To: Fujii Masao <masao.fujii@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>
Subject: Re: adding wait_start column to pg_locks
In-Reply-To: <1df88660-6f08-cc6e-b7e2-f85296a2bdab@oss.nttdata.com>
References: <a96013dc51cdc56b2a2b84fa8a16a993@oss.nttdata.com>
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 <a5bfd1c2f2fb21eb7ea5ddec2e094430@oss.nttdata.com>
 <8002219d-b999-12fa-2327-49afe40e5fbb@oss.nttdata.com>
 <88c451c7-2729-4329-fbae-6ed7664adea1@oss.nttdata.com>
 <f9153182845e5584b954ab6f03514d13@oss.nttdata.com>
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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