Re: Get rid of WALBufMappingLock

14.02.2025 13:24, Alexander Korotkov пишет:
> On Fri, Feb 14, 2025 at 11:45 AM Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com> wrote:
>> On Fri, 14 Feb 2025 at 00:59, Alexander Korotkov <aekorotkov(at)gmail(dot)com> wrote:
>>> On Thu, Feb 13, 2025 at 6:39 PM Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com> wrote:
>>>> On Thu, 13 Feb 2025 at 14:08, Alexander Korotkov <aekorotkov(at)gmail(dot)com> wrote:
>>>>>
>>>>> On Thu, Feb 13, 2025 at 11:45 AM Yura Sokolov <y(dot)sokolov(at)postgrespro(dot)ru> wrote:
>>>>>> 13.02.2025 12:34, Alexander Korotkov пишет:
>>>>>>> On Wed, Feb 12, 2025 at 8:16 PM Yura Sokolov <y(dot)sokolov(at)postgrespro(dot)ru> wrote:
>>>>>>>> 08.02.2025 13:07, Alexander Korotkov пишет:
>>>>>>>>> On Fri, Feb 7, 2025 at 1:39 PM Alexander Korotkov <aekorotkov(at)gmail(dot)com> wrote:
>>>>>>>>>> Good, thank you. I think 0001 patch is generally good, but needs some
>>>>>>>>>> further polishing, e.g. more comments explaining how does it work.
>>>>>>>>
>>>>>>>> I tried to add more comments. I'm not good at, so recommendations are welcome.
>>>>>>>>
>>>>>>>>> Two things comes to my mind worth rechecking about 0001.
>>>>>>>>> 1) Are XLogCtl->InitializeReserved, XLogCtl->InitializedUpTo and
>>>>>>>>> XLogCtl->xlblocks always page-aligned? Because algorithm seems to be
>>>>>>>>> sensitive to that. If so, I would propose to explicitly comment that
>>>>>>>>> and add corresponding asserts.
>>>>>>>>
>>>>>>>> They're certainly page aligned, since they are page borders.
>>>>>>>> I added assert on alignment of InitializeReserved for the sanity.
>>>>>>>>
>>>>>>>>> 2) Check if there are concurrency issues between
>>>>>>>>> AdvanceXLInsertBuffer() and switching to the new WAL file.
>>>>>>>>
>>>>>>>> There are no issues:
>>>>>>>> 1. CopyXLogRecordToWAL for isLogSwitch follows same protocol, ie uses
>>>>>>>> GetXLogBuffer to zero-out WAL page.
>>>>>>>> 2. WALINSERT_SPECIAL_SWITCH forces exclusive lock on all insertion locks,
>>>>>>>> so switching wal is not concurrent. (Although, there is no need in this
>>>>>>>> exclusiveness, imho.)
>>>>>>>
>>>>>>> Good, thank you. I've also revised commit message and comments.
>>>>>>>
>>>>>>> But I see another issue with this patch. In the worst case, we do
>>>>>>> XLogWrite() by ourselves, and it could potentially could error out.
>>>>>>> Without patch, that would cause WALBufMappingLock be released and
>>>>>>> XLogCtl->InitializedUpTo not advanced. With the patch, that would
>>>>>>> cause other processes infinitely waiting till we finish the
>>>>>>> initialization.
>>>>>>>
>>>>>>> Possible solution would be to save position of the page to be
>>>>>>> initialized, and set it back to XLogCtl->InitializeReserved on error
>>>>>>> (everywhere we do LWLockReleaseAll()). We also must check that on
>>>>>>> error we only set XLogCtl->InitializeReserved to the past, because
>>>>>>> there could be multiple concurrent failures. Also we need to
>>>>>>> broadcast XLogCtl->InitializedUpToCondVar to wake up waiters.
>>>>>>
>>>>>> The single place where AdvanceXLInsertBuffer is called outside of critical
>>>>>> section is in XLogBackgroundFlush. All other call stacks will issue server
>>>>>> restart if XLogWrite will raise error inside of AdvanceXLInsertBuffer.
>>>>>>
>>>>>> XLogBackgroundFlush explicitely avoids writing buffers by passing
>>>>>> opportunistic=true parameter.
>>>>>>
>>>>>> Therefore, error in XLogWrite will not cause hang in AdvanceXLInsertBuffer
>>>>>> since server will shutdown/restart.
>>>>>>
>>>>>> Perhaps, we just need to insert `Assert(CritSectionCount > 0);` before call
>>>>>> to XLogWrite here?
>>>>>
>>>>> You're correct. I just reflected this in the next revision of the patch.
>>>>
>>>> I've looked at the patchset v6.
>>>
>>> Oh, sorry, I really did wrong. I've done git format-patch for wrong
>>> local branch for v5 and v6. Patches I've sent for v5 and v6 are
>>> actually the same as my v1. This is really pity. Please, find the
>>> right version of patchset attached.
>>
>> I've rechecked v7. In v6 a proposal from [1] was not reflected. Now it
>> landed in v7.
>>
>> Other changes are not regarding code behavior. The things from my
>> previous review that still could apply to v7:
>>
>> For 0001:
>>
>> Comment change proposed:
>> "lock-free with cooperation with" -> "lock-free accompanied by changes
>> to..." (maybe other variant)
>
> Good catch. I've rephrased this comment even more.
>
>> I propose a new define:
>> #define FirstValidXLogRecPtr 1
>> While FirstValidXLogRecPtr = InvalidXLogRecPtr + 1 is true in the code
>> that has no semantical meaning and it's better to avoid using direct
>> arithmetics to relate meaning of FirstValidXLogRecPtr from
>> InvalidXLogRecPtr.
>
> Makes sense, but I'm not sure if this change is required at all. I've
> reverted this to the state of master, and everything seems to work.
>
>> For 0002 both comments proposals from my message applied to v6 apply
>> to v7 as well
>
> Thank you for pointing. For now, I'm concentrated on improvements on
> 0001. Probably Yura could work on your notes to 0002.

I wrote good commit message for 0002 with calculated probabilities and
simple Ruby program which calculates them to explain choice of 2
conditional attempts. (At least I hope the message is good). And added
simple comment before `int attempts = 2;`

Also I simplified 0002 a bit to look a bit prettier (ie without goto), and
added static assert on NUM_XLOGINSERT_LOCKS being power of 2.

(0001 patch is same as for v8)

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regards
Yura Sokolov aka funny-falcon