Re: gc buffer busy from the same instance??
Date: Sat, 9 May 2009 13:43:55 +0100
Message-ID: <6K6dnaMWuOcX55jXnZ2dnUVZ8oqdnZ2d_at_bt.com>
<stevedhoward_at_gmail.com> wrote in message news:fe3978e2-5fde-4cbd-ba1f-7c48328b80c4_at_o14g2000vbo.googlegroups.com... On May 8, 3:10 pm, Steve Howard <stevedhow..._at_gmail.com> wrote:
> Hi All,
>
> 10.2.0.4 EE on SLES 10
>
> We have seen fairly significant waits recently on this during a large
> load recently. We know which query is causing it and are submitting a
> fix, but my question is the following:
>
> When I select from gv$session, I see session x on instance 2 waiting
> on "db file sequential read" for a given file and block number. I see
> session y, also on instance 2, waiting on "gc buffer busy" for the
> same file and block. These are the only two sessions going after this
> block.
>
> I can understand (and would expect) this if session y was on instance
> 3, or whatever. However, since both sessions are on the same
> instance, why is this not either recorded as "read by other session"
> or "buffer busy waits" event?
>
> Thanks,
>
> Steve
I found this in the docs, which to me still doesn't explain it...
http://download.oracle.com/docs/cd/B19306_01/rac.102/b14197/monitor.htm#CFAGAAGD
/
High concurrency is instead evidenced by the existence of the gc buffer busy event. This event indicates that the block was pinned or held up by a session on a remote instance. It can also indicate that a session on the same instance has already requested the block, which in either case is in transition between instances and the current session needs to wait behind it.
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How is a session on the same instance requesting a block the same as a transition between instances??
I'm sure the diagnostics pack (at least ADDM) uses the wait classes for its recommendations. If it sees this as a cluster class event, vs I/O, it may recommend different things (sometimes incorrectly)
I will open an SR for clarification and report back.
Here's a possible scenario:
Session 1 on node 1 wants the block in current mode
Session A on node 2 is changing it causing session 1 to go into a gc buffer busy wait
Session 2 on node 1 wants the same block in current mode and goes into gc buffer busy wait as well
Session A releases the block on node 2, but for various reasons it has to be passed by disk, not by the interconnect, so session 1 terminates its gc buffer busy wait and goes into a db file sequential read.
-- Regards Jonathan Lewis http://jonathanlewis.wordpress.com Author: Cost Based Oracle: Fundamentals http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html The Co-operative Oracle Users' FAQ http://www.jlcomp.demon.co.uk/faq/ind_faq.htmlReceived on Sat May 09 2009 - 07:43:55 CDT