Re: Direct SGA access problem because of structs' changing start addresses (e.g., x$ksmsp)

Frits Hoogland

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On 07 Jan 2014, at 23:33, keydana_at_gmx.de wrote:

> Hi,
>
> I'm trying to write some Haskell code for direct SGA access of - preferredly, as of today - x$ksmsp (sql queries against this table being expensive, possibly...), and for that I'm following the approach from Kyle Hailey, as detailed in the Oracle Wait Interface book.
>
> So I need the struct's start address, but unfortunately, when I query x$ksmsp several times in a row - even from the same sqlplus process - I get different results every time... (Actually, this seems to be the case with most fixed tables, apart from those "fixed" fixed ones as x$ksuse...)
This seems very logical to me. This view externalises the shared pool administration (if memory serves me well), which has a lot of stuff going on all the time. You even might think twice about doing this on a real life, busy, system. I would have to look it up, but I think this view holds latches to get a more or less consistent view, which is the reason this view can take some time (acquiring the latches).

The X$ views are essentially Oracle memory structures externalised as database tables. Because the data is so transient (you are watching memory areas, hash tables, arrays, etc. of a running program), the design of this views is the tables can be inconsistent (it would hang your system to lock everything down for a consistent version).

This means that even if you dump the shared pool of a running system a few times from oradebug, you will see differences too, because all the processes keep on doing stuff.

>
> It'd be great if anyone had advice, or an idea, what I could do to get this working all the same (and as to what the reason might be?)
Becasue the X$ tables are not tables, but externalised views on memory structures of a running program.
> It ought to be possible somehow to do this, as I've tried oradebug direct access with x$ksmsp and it worked (of course, it runs in sqlplus, so I don't know...)
>
> Thanks a lot in advance!
>
> Sigrid--
> http://www.freelists.org/webpage/oracle-l
>
>

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