Subject: Re: adding wait_start column to pg_locks
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>
 <20210101214930.GH25152@telsasoft.com>
 <9e958478d8bcea814e7ce16511a18912@oss.nttdata.com>
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 <CA+TgmoYowQhMhT74AsxDib6e3LFPGvXVxVOUdP0-hMNY9c1wgw@mail.gmail.com>
 <CAB8KJ=idS0m_+65d2ujjjZj5DSwpYYV3XoHMwr-UZ-OFs3Mb4Q@mail.gmail.com>
 <23d39ee9c31643fad8f9ba9c5cf3aaf4@oss.nttdata.com>
 <0fd375a53e306566cea7f451cc8cfcde@oss.nttdata.com>
 <7b1bb07a-73ec-1fcc-2f65-41101adf0080@oss.nttdata.com>
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From: Fujii Masao <masao.fujii@oss.nttdata.com>
Message-ID: <990e5d4b-075f-101f-aec5-bb44f9b30550@oss.nttdata.com>
Date: Tue, 9 Feb 2021 18:13:37 +0900
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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.

Regards,

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