Re: [GENERAL] Why SyncOneBuffer does not called frequently?

[Xiong He]

HI, 

I just debug the code, find that the following stack will call the 
SyncOneBuffer.

>    postgres.exe!SyncOneBuffer(int buf_id=2, char skip_recently_used=0)  Line 
> 1640    C
     postgres.exe!BufferSync(int flags=64)  Line 1284 + 0xb bytes    C
     postgres.exe!CheckPointBuffers(int flags=64)  Line 1801 + 0x9 bytes    C
     postgres.exe!CheckPointGuts(XLogRecPtr checkPointRedo={...}, int flags=64) 
 Line 8129 + 0x9 bytes    C
     postgres.exe!CreateCheckPoint(int flags=64)  Line 7977 + 0x11 bytes    C
     postgres.exe!CheckpointerMain()  Line 505 + 0x9 bytes    C
     postgres.exe!AuxiliaryProcessMain(int argc=2, char * * argv=0x002f6fb8)  
Line 429    C
     postgres.exe!SubPostmasterMain(int argc=4, char * * argv=0x002f6fb0)  Line 
4136 + 0x13 bytes    C
     postgres.exe!main(int argc=4, char * * argv=0x002f6fb0)  Line 176 + 0xd 
bytes    C
     postgres.exe!__tmainCRTStartup()  Line 582 + 0x19 bytes    C
     postgres.exe!mainCRTStartup()  Line 399    C

This is the check point background process.



------------------
Thanks&Regards,
Xiong He





 




------------------ Original ------------------
From:  "????"<luckyjack...@gmail.com>;
Date:  Wed, Oct 31, 2012 04:53 PM
To:  "pgsql-general"<pgsql-general@postgresql.org>; 

Subject:  [GENERAL] Why SyncOneBuffer does not called frequently?



Hi all:


I am trying to understand when the bgwriter is written.


I thought that the  bgwriter.c's calling turn is:


BackgroundWriterMain ->BgBufferSync-> SyncOneBuffer
 

And In my postgresql.conf , the bgwriter_delay=200ms.


I did the following:


postgres=# select * from testtab;
 id |  val  
----+-------
   1 | 12345
(1 row)


postgres=# update testtab set val='54321' where id=1;
UPDATE 1
postgres=# select * from testtab;
 id |  val  
----+-------
   1 | 54321
(1 row)


postgres=# 


Now I can say the buffer is dirty ,right?


I wait for a few minutes, I can found bgwriter's BackgroundWriterMain called 
BgBufferSync many times.
 

But I can't find BgBufferSync really call SyncOneBuffer to put the dirty data 
todisk.


Untill I close the postgres process, I can find the SyncOneBuffer is called for 
many times.
 

My question is: 
Why even there are dirty buffer(s), the SyncOneBuffer is still not called?  
Is it violating the background writer's purpose? 


Or the flushing to disk will be done untill  the amount of block/buffer is 
satisfied? If so , what is it?
 

Thanks in advance for any help