Re: Get rid of WALBufMappingLock

07.02.2025 14:02, Yura Sokolov пишет:
> 07.02.2025 01:26, Alexander Korotkov пишет:
>> Hi!
>>
>> On Sun, Jan 19, 2025 at 2:11 AM Yura Sokolov <y(dot)sokolov(at)postgrespro(dot)ru> wrote:
>>>
>>> During discussion of Increasing NUM_XLOGINSERT_LOCKS [1], Andres Freund
>>> used benchmark which creates WAL records very intensively. While I this
>>> it is not completely fair (1MB log records are really rare), it pushed
>>> me to analyze write-side waiting of XLog machinery.
>>>
>>> First I tried to optimize WaitXLogInsertionsToFinish, but without great
>>> success (yet).
>>>
>>> While profiling, I found a lot of time is spend in the memory clearing
>>> under global WALBufMappingLock:
>>>
>>> MemSet((char *) NewPage, 0, XLOG_BLCKSZ);
>>>
>>> It is obvious scalability bottleneck.
>>>
>>> So "challenge was accepted".
>>>
>>> Certainly, backend should initialize pages without exclusive lock. But
>>> which way to ensure pages were initialized? In other words, how to
>>> ensure XLogCtl->InitializedUpTo is correct.
>>>
>>> I've tried to play around WALBufMappingLock with holding it for a short
>>> time and spinning on XLogCtl->xlblocks[nextidx]. But in the end I found
>>> WALBufMappingLock is useless at all.
>>>
>>> Instead of holding lock, it is better to allow backends to cooperate:
>>> - I bound ConditionVariable to each xlblocks entry,
>>> - every backend now checks every required block pointed by
>>> InitializedUpto was successfully initialized or sleeps on its condvar,
>>> - when backend sure block is initialized, it tries to update
>>> InitializedUpTo with conditional variable.
>>
>> Looks reasonable for me, but having ConditionVariable per xlog buffer
>> seems overkill for me. Find an attached revision, where I've
>> implemented advancing InitializedUpTo without ConditionVariable.
>> After initialization of each buffer there is attempt to do CAS for
>> InitializedUpTo in a loop. So, multiple processes will try to advance
>> InitializedUpTo, they could hijack initiative from each other, but
>> there is always a leader which will finish the work.
>>
>> There is only one ConditionVariable to wait for InitializedUpTo being advanced.
>>
>> I didn't benchmark my version, just checked that tests passed.
>
> Good day, Alexander.

Seems I was mistaken twice.

> I've got mixed but quite close result for both approaches (single or many
> ConditionVariable) on the notebook. Since I have no access to larger
> machine, I can't prove "many" is way better (or discover it worse).

1. "many condvars" (my variant) is strictly worse with num locks = 64 and
when pg_logical_emit_message emits just 1kB instead of 1MB.

Therefore, "single condvar" is strictly better.

> Given patch after cleanup looks a bit smaller and clearer, I agree to keep
> just single condition variable.
>
> Cleaned version is attached.
>
> I've changed condition for broadcast a bit ("less" instead "not equal"):
> - buffer's border may already go into future,
> - and then other backend will reach not yet initialized buffer and will
> broadcast.

2. I've inserted abort if "buffer's border went into future", and wasn't
able to trigger it.

So I returned update-loop's body to your variant.

-------
regards
Yura Sokolov aka funny-falcon