Re: BufferUsage counters' values have changed

Nazir, Andres, thank you both for help!

On Wed, Sep 13, 2023 at 10:10 PM Andres Freund <andres(at)anarazel(dot)de> wrote:

> On 2023-09-13 16:04:00 +0300, Nazir Bilal Yavuz wrote:
> > local_blks_read became zero because:
> > 1_ One more cache hit. It was supposed to be local_blks_read but it is
> > local_blks_hit now. This is an assumption, I didn't check this deeply.
> > 2_ Before fcdda1e4b5, there was one local_blks_read coming from
> > buf = ReadBufferExtended(rel, VISIBILITYMAP_FORKNUM, blkno,
> > RBM_ZERO_ON_ERROR, NULL) in freespace.c -> ReadBuffer_common() ->
> > pgBufferUsage.local_blks_read++.
> > But buf = ReadBufferExtended(rel, VISIBILITYMAP_FORKNUM, blkno,
> > RBM_ZERO_ON_ERROR, NULL) is moved into the else case, so it didn't
> > called and local_blks_read isn't incremented.
>
> That imo is a legitimate difference / improvement. The read we previously
> did
> here was unnecessary.
>
>
> > local_blks_written is greater because of the combination of fcdda1e4b5
> > and 00d1e02be2.
> > In PG_15:
> > RelationGetBufferForTuple() -> ReadBufferBI(P_NEW, RBM_ZERO_AND_LOCK)
> > -> ReadBufferExtended() -> ReadBuffer_common() ->
> > pgBufferUsage.local_blks_written++; (called 5 times) [0]
> > In PG_16:
> > 1_ 5 of the local_blks_written is coming from:
> > RelationGetBufferForTuple() -> RelationAddBlocks() ->
> > ExtendBufferedRelBy() -> ExtendBufferedRelCommon() ->
> > ExtendBufferedRelLocal() -> pgBufferUsage.local_blks_written +=
> > extend_by; (extend_by is 1, this is called 5 times) [1]
> > 2_ 3 of the local_blks_written is coming from:
> > RelationGetBufferForTuple() -> RecordAndGetPageWithFreeSpace() ->
> > fsm_set_and_search() -> fsm_readbuf() -> fsm_extend() ->
> > ExtendBufferedRelTo() -> ExtendBufferedRelCommon() ->
> > ExtendBufferedRelLocal() -> pgBufferUsage.local_blks_written +=
> > extend_by; (extend_by is 3, this is called 1 time) [2]
> >
> > I think [0] is the same path as [1] but [2] is new. 'fsm extends'
> > wasn't counted in local_blks_written in PG_15. Calling
> > ExtendBufferedRelTo() from fsm_extend() caused 'fsm extends' to be
> > counted in local_blks_written. I am not sure which one is correct.
>
> I think it's correct to count the fsm writes here. The pg_stat_statement
> columns aren't specific to the main relation for or such... If anything it
> was
> a bug to not count them before.
>

Best regards,
Karina Litskevich
Postgres Professional: http://postgrespro.com/