Re: Seeing the same session_id used on different nodes alot in RAC 11g
- From: Tanel Poder <tanel@xxxxxxxxxxxxxx>
- To: keydana@xxxxxx
- Date: Fri, 27 Feb 2015 13:13:15 -0600
Yep the SID is the *slot* number in v$session's underlying ksuse array just
like PID is the *slot* number of v$process underlying ksupr array. These
two arrays are not segmented as far as I know (otherwise they'd show up in
x$ksmdd). This is the reason why you may need to
increase _ksmg_granule_size to avoid ORA-64 when configuring tens of
thousands of sessions - the entire array must fit into the linear address
range of a single granule.
Oracle (and any regular C program) would address the array elements this
way: array_start_addr + (SID * slot size) + element_offset_in_slot. With
segmented arrays it would be less trivial as you'll need to determine in
which segment the slot would reside and then look up that segment's
starting address.
An example of ORA-64 from past:
https://www.freelists.org/post/oracle-l/ORA00064-object-is-too-large-to-allocate-on-this-OS-116352000,3
--
Tanel Poder
http://blog.tanelpoder.com
On Fri, Feb 27, 2015 at 11:46 AM, keydana@xxxxxx <keydana@xxxxxx> wrote:
> Thanks Stefan, it’s a shame I forgot about this (time for yet another read
> of the book!) :-)
>
>
> > Am 27.02.2015 um 18:43 schrieb Stefan Koehler <contact@xxxxxxxx>:
> >
> > Hi Sigrid,
> > Jonathan Lewis described the difference and its functionality very well
> in his book "Oracle Core: Essential Internals for DBAs and Developers". I am
> > just quoting from page 59.
> >
> >
> > "An array is essentially a list of objects of the same shape and size,
> and since all the objects are the same size, it’s easy to walk through the
> > array looking at each object in turn. For example, x$ksuse (the
> structure for user sessions reported through v$session) is a fixed array
> with rows of
> > 11,360 bytes in Oracle 11.2.0.2 on 32-bit Windows. Oracle need only
> record the starting position of the array, and the code can work out the
> starting
> > position of any row in the array by simple arithmetic.
> >
> > In some cases the array is a “segmented” array, which means Oracle
> allocates an initial chunk of memory for a fixed number of array items, but
> may
> > allocate more chunks dynamically as needed. In this case Oracle needs to
> record the addresses of the starting point of each chunk—which might mean
> > that it keeps a little list of the chunks, or that each chunk includes a
> little information about the next chunk in the list. The structure x$ktatl
> > (the entry labeled temporary_table_locks in v$resource_limit) is such a
> case. In a little test I ran against a 10g instance, this started life as an
> > array with 16 entries of 144 bytes each, but then added chunks of 16
> rows at a time, with each chunk allocated fairly randomly in memory. In a
> similar
> > fashion, although x$ksqrs and x$ksqeq start with a much larger array
> size they can grow by chunks of 32 rows at a time."
> >
> >
> > So segmented means that it may allocates chunks dynamically and it needs
> to track the pointer / address to the next randomly distributed memory chunk
> > (not only between array entries).
> >
> > Best Regards
> > Stefan Koehler
> >
> > Freelance Oracle performance consultant and researcher
> > Homepage: http://www.soocs.de
> > Twitter: @OracleSK
> >
> >> "keydana@xxxxxx" <keydana@xxxxxx> hat am 27. Februar 2015 um 18:24
> geschrieben:
> >>
> >> Wanted to know for a long time, this is the opportunity to ask, - what
> exactly do you mean by that (segmented array signifying „list is in-built“)?
> >>
> >> Does it mean the structs (the elements of the array) have pointers to
> the previous & next element? (If yes, I wouldn’t have guessed that from the
> >> expression „segmented array“)
> >>
> >> thanks!
> >> Sigrid
>
> --
> //www.freelists.org/webpage/oracle-l
>
>
>
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