Re: BufferUsage counters' values have changed

Hi,

On Mon, 11 Sept 2023 at 14:28, Karina Litskevich
<litskevichkarina@gmail.com> wrote:
>
> Hi hackers,
>
> I noticed that BufferUsage counters' values are strangely different for the
> same queries on REL_15_STABLE and REL_16_STABLE. For example, I run
>
> CREATE EXTENSION pg_stat_statements;
> CREATE TEMP TABLE test(b int);
> INSERT INTO test(b) SELECT generate_series(1,1000);
> SELECT query, local_blks_hit, local_blks_read, local_blks_written,
>        local_blks_dirtied, temp_blks_written FROM pg_stat_statements;
>
> and output on REL_15_STABLE contains
>
> query              | INSERT INTO test(b) SELECT generate_series($1,$2)
> local_blks_hit     | 1005
> local_blks_read    | 2
> local_blks_written | 5
> local_blks_dirtied | 5
> temp_blks_written  | 0
>
> while output on REL_16_STABLE contains
>
> query              | INSERT INTO test(b) SELECT generate_series($1,$2)
> local_blks_hit     | 1006
> local_blks_read    | 0
> local_blks_written | 0
> local_blks_dirtied | 5
> temp_blks_written  | 8
>
>
> I found a bug that causes one of the differences. Wrong counter is incremented
> in ExtendBufferedRelLocal(). The attached patch fixes it and should be applied
> to REL_16_STABLE and master. With the patch applied output contains

Nice finding! I agree, it should be changed.

> query              | INSERT INTO test(b) SELECT generate_series($1,$2)
> local_blks_hit     | 1006
> local_blks_read    | 0
> local_blks_written | 8
> local_blks_dirtied | 5
> temp_blks_written  | 0
>
>
> I still wonder why local_blks_written is greater than it was on REL_15_STABLE,
> and why local_blks_read became zero. These changes are caused by fcdda1e4b5.
> This code is new to me, and I'm still trying to understand whether it's a bug
> in computing the counters or just changes in how many blocks are read/written
> during the query execution. If anyone can help me, I would be grateful.

I spent some time on it:

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.

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 hope these help.

Regards,
Nazir Bilal Yavuz
Microsoft