Re: [HACKERS] AdvanceXLInsertBuffer vs. WAL segment compressibility

On 06/26/2017 04:20 AM, Chapman Flack wrote:
> I notice CopyXLogRecordToWAL contains this loop (in the case where
> the record being copied is a switch):
>
> while (CurrPos < EndPos)
> {
>     /* initialize the next page (if not initialized already) */
>     WALInsertLockUpdateInsertingAt(CurrPos);
>     AdvanceXLInsertBuffer(CurrPos, false);
>     CurrPos += XLOG_BLCKSZ;
> }
>
> in which it calls, one page at a time, AdvanceXLInsertBuffer, which contains
> its own loop able to do a sequence of pages. A comment explains why:
>
> /*
>  * We do this one page at a time, to make sure we don't deadlock
>  * against ourselves if wal_buffers < XLOG_SEG_SIZE.
>  */
>
> I want to make sure I understand what the deadlock potential is
> in this case. AdvanceXLInsertBuffer will call WaitXLogInsertionsToFinish
> before writing any dirty buffer, and we do hold insertion slot locks
> (all of 'em, in the case of a log switch, because that makes
> XlogInsertRecord call WALInsertLockAcquireExclusive instead of just
> WALInsertLockAcquire for other record types).
>
> Does not the fact we hold all the insertion slots exclude the possibility
> that any dirty buffer (preceding the one we're touching) needs to be checked
> for in-flight insertions?

Hmm. That's not the problem, though. Imagine that instead of the loop 
above, you do just:

WALInsertLockUpdateInsertingAt(CurrPos);
AdvanceXLInsertBuffer(EndPos, false);

AdvanceXLInsertBuffer() will call XLogWrite(), to flush out any pages 
before EndPos, to make room in the wal_buffers for the new pages. Before 
doing that, it will call WaitXLogInsertionsToFinish() to wait for any 
insertions to those pages to be completed. But the backend itself is 
advertising the insertion position CurrPos, and it will therefore wait 
for itself, forever.

> I've been thinking along the lines of another parameter to
> AdvanceXLInsertBuffer to indicate when the caller is exactly this loop
> filling out the tail after a log switch (originally, to avoid filling
> in page headers). It now seems to me that, if AdvanceXLInsertBuffer
> has that information, it could also be safe for it to skip the
> WaitXLogInsertionsToFinish in that case. Would that eliminate the
> deadlock potential, and allow the loop in CopyXLogRecordToWAL to be
> replaced with a single call to AdvanceXLInsertBuffer and a single
> WALInsertLockUpdateInsertingAt ?
>
> Or have I overlooked some other subtlety?

The most straightforward solution would be to just clear each page with 
memset() in the loop. It's a bit wasteful to clear the page again, just 
after AdvanceXLInsertBuffer() has initialized it, but this isn't 
performance-critical.

- Heikki