From: Fujii Masao <masao.fujii@oss.nttdata.com>
To: torikoshia <torikoshia@oss.nttdata.com>
Cc: Ian Lawrence Barwick <barwick@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: adding wait_start column to pg_locks
Date: Mon, 22 Feb 2021 18:27:36 +0900
Message-ID: <0630dd64-a97f-28d1-5c5d-640cbfc293cc@oss.nttdata.com> (raw)
In-Reply-To: <62fe634bce475fb7e38e6fb3a2fff124@oss.nttdata.com>
References: <a96013dc51cdc56b2a2b84fa8a16a993@oss.nttdata.com>
<a5bfd1c2f2fb21eb7ea5ddec2e094430@oss.nttdata.com>
<8002219d-b999-12fa-2327-49afe40e5fbb@oss.nttdata.com>
<88c451c7-2729-4329-fbae-6ed7664adea1@oss.nttdata.com>
<f9153182845e5584b954ab6f03514d13@oss.nttdata.com>
<ef0dd522-9096-a60f-a0fa-5f0a0fdbafbd@oss.nttdata.com>
<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>
<64102638-7fc3-1941-12f1-e139e3463c03@oss.nttdata.com>
<067d5597e5de82fe47e5716991ab65c4@oss.nttdata.com>
<337d5311-6113-15e0-bd85-90b47b55f5ae@oss.nttdata.com>
<655f1a5b-4fa8-53ba-c3e2-8cee31587d37@oss.nttdata.com>
<1df88660-6f08-cc6e-b7e2-f85296a2bdab@oss.nttdata.com>
<62fe634bce475fb7e38e6fb3a2fff124@oss.nttdata.com>
On 2021/02/18 16:26, torikoshia wrote:
> 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.
Thanks for the review and test! I pushed the patch.
Regards,
--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-hackers@postgresql.org
Cc: masao.fujii@oss.nttdata.com, torikoshia@oss.nttdata.com, barwick@gmail.com, robertmhaas@gmail.com, pryzby@telsasoft.com
Subject: Re: adding wait_start column to pg_locks
In-Reply-To: <0630dd64-a97f-28d1-5c5d-640cbfc293cc@oss.nttdata.com>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox