Re: BufferUsage counters' values have changed
От | Andres Freund |
---|---|
Тема | Re: BufferUsage counters' values have changed |
Дата | |
Msg-id | 20230913191030.hp7ooi7wcnxgatsi@awork3.anarazel.de обсуждение исходный текст |
Ответ на | Re: BufferUsage counters' values have changed (Nazir Bilal Yavuz <byavuz81@gmail.com>) |
Ответы |
Re: BufferUsage counters' values have changed
|
Список | pgsql-hackers |
Hi, On 2023-09-13 16:04:00 +0300, Nazir Bilal Yavuz wrote: > 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. 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. Thanks for looking into this. Greetings, Andres Freund
В списке pgsql-hackers по дате отправления: