Neil Conway <[EMAIL PROTECTED]> writes: > In the BufferDesc struct, there seem to be two ways to mark a buffer > page as dirty: setting the BM_DIRTY bit mask in the 'flags' field of the > struct, and setting the 'cntxDirty' field to true. What is the > difference between these two indications of a page's dirtiness? > Or, more to the point, is there a reason we have two ways to do what > looks like the same thing?
I believe the reason for this is that you are allowed to set cntxDirty to TRUE while holding (only) the context lock on the buffer, while messing with the buffer flags word requires holding (only) the BufMgrLock. Getting rid of cntxDirty would mean additional grabbings of the BufMgrLock when we want to mark buffers dirty. The real solution to this is probably to rethink the rules for locking in the buffer manager. I've thought for some time that the BufMgrLock is a system-wide bottleneck; if we could replace it by finer-grain locks, and in particular use the per-buffer locks for operations affecting just the state of a single buffer, we'd be ahead of the game. > BTW, I'd like to remove the behavior that LockBuffer(buf, EXCLUSIVE) > automatically marks the page as dirty. Yeah. This has been discussed before, see the archives: http://archives.postgresql.org/pgsql-hackers/2002-11/msg00488.php http://archives.postgresql.org/pgsql-hackers/2002-11/msg00679.php http://archives.postgresql.org/pgsql-hackers/2002-11/msg00512.php regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 8: explain analyze is your friend
