Re: bufmgr code question

Is there a TODO here?

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Tom Lane wrote:
> Neil Conway <neilc(at)samurai(dot)com> 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
>
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