Re: [RFC] Lock-free XLog Reservation from WAL - Mailing list pgsql-hackers

02.01.2025 10:05, Zhou, Zhiguo wrote:
> Hi all,
> 
> I am reaching out to solicit your insights and comments on a recent proposal regarding the "Lock-free XLog
Reservationfrom WAL." We have identified some challenges with the current WAL insertions, which require space
reservationsin the WAL buffer which involve updating two shared-memory statuses in XLogCtlInsert: CurrBytePos (the
startposition of the current XLog) and PrevBytePos (the prev-link to the previous XLog). Currently, the use of
XLogCtlInsert.insertpos_lckensures consistency but introduces lock contention, hindering the parallelism of XLog
insertions.
> 
> To address this issue, we propose the following changes:
> 
> 1. Removal of PrevBytePos: This will allow increments of the CurrBytePos (a single uint64 field) to be implemented
withan atomic operation (fetch_add).
 
> 2. Updating Prev-Link of next XLog: Based on the fact that the prev-link of the next XLog always points to the head
ofthe current Xlog,we will slightly exceed the reserved memory range of the current XLog to update the prev-link of the
nextXLog, regardless of which backend acquires the next memory space. The next XLog inserter will wait until its
prev-linkis updated for CRC calculation before starting its own XLog copy into the WAL.
 
> 3. Breaking Sequential Write Convention: Each backend will update the prev-link of its next XLog first, then return
tothe header position for the current log insertion. This change will reduce the dependency of XLog writes on previous
ones(compared with the sequential writes).
 
> 4. Revised GetXLogBuffer: To support #3, we need update this function to separate the LSN it intends to access from
theLSN it expects to update in the insertingAt field.
 
> 5. Increase NUM_XLOGINSERT_LOCKS: With the above changes, increasing NUM_XLOGINSERT_LOCKS, for example to 128, could
effectivelyenhance the parallelism.
 
> 
> The attached patch could pass the regression tests (make check, make check-world), and in the performance test of
thisPOC on SPR (480 vCPU) shows that this optimization could help the TPCC benchmark better scale with the core count
andas a result the performance with full cores enabled could be improved by 2.04x.
 
> 
> Before we proceed with further patch validation and refinement work, we are eager to hear the community's thoughts
andcomments on this optimization so that we can confirm our current work aligns with expectations.
 

Good day, Zhiguo.

Idea looks great.

Minor issue:
- you didn't remove use of `insertpos_lck` from `ReserveXLogSwitch`.

I initially thought it became un-synchronized against 
`ReserveXLogInsertLocation`, but looking closer I found it is 
synchronized with `WALInsertLockAcquireExclusive`.
Since there are no other `insertpos_lck` usages after your patch, I 
don't see why it should exists and be used in `ReserveXLogSwitch`.

Still I'd prefer to see CAS loop in this place to be consistent with 
other non-locking access. And it will allow to get rid of 
`WALInsertLockAcquireExclusive`, (though probably it is not a big issue).

Major issue:
- `SetPrevRecPtr` and `GetPrevRecPtr` do non-atomic write/read with on 
platforms where MAXALIGN != 8 or without native 64 load/store. Branch 
with 'memcpy` is rather obvious, but even pointer de-referencing on 
"lucky case" is not safe either.

I have no idea how to fix it at the moment.

Readability issue:
- It would be good to add `Assert(ptr >= upto)` into `GetXLogBuffer`.
I had hard time to recognize `upto` is strictly not in the future.
- Certainly, final version have to have fixed and improved comments. 
Many patch's ideas are strictly non-obvious. I had hard time to 
recognize patch is not a piece of ... (excuse me for the swear sentence).

Indeed, patch is much better than it looks on first sight.
I came with alternative idea yesterday, but looking closer to your patch 
today I see it is superior to mine (if atomic access will be fixed).

----

regards,
Yura Sokolov aka funny-falcon