IDS Forum

Re: shared memory size limit on 32-bit vs 64 bit I

Posted By: Martin Fuerderer
Date: Thursday, 6 May 2010, at 8:12 a.m.

In Response To: shared memory size limit on 32-bit vs 64 bit IDS (FAN YANG)

Hi,

a "word" is used to address the memory. Hence the size of a
word determines how much memory is adressable. This
addressable memory also is referred to as "address space".
The size of a word on a 32-bit architecture is 32 bit, on a
64-bit architecture it is 64 bit. (Actually this size of the word
is giving the names "32-bit" or "64-bit".) With a 32-bit word,
4 GB of memory can be addressed. With a 64-bit word, ...
... much more than normally physically available (in these days)
can be addressed. This concept of address space is valid for
each process running on a system. So much for the general theory.

In practice however, the address space get's fragmented by
different pieces (segments) of memory used for different purposes.
Usually there is a heap, a stack, somewhere are shared libraries,
maybe there is a "text segment" (containing the programs code),
etc. All these things are mapped into a processes address space,
at different places within this space. So the "free memory" that
can be used by an application is limited by how much these other
things take up - and by how much the remaining space is fragmented.
And this is where in general the OS specifics come into the picture.
Each OS has a different way of doing the reservation for and mapping
of these different segments. Often both, reservation and mapping can
somehow be influenced or configured, sometimes per process,
sometimes system wide (e.g. by kernel parameters).

IDS specific in this picture is, that IDS mostly (almost exclusively)
uses shared memory. This is also mapped to segments in the
address space of a process. The config parameter SHMBASE
determines, where in the address space the first shared memory
segment will be mapped to. Following shared memory segments
are mapped consecutively whenever possible. But in the end the
OS decides about this mapping. However, one shared memory
segment of IDS (see also "onstat -g seg") can consist of several
OS shared memory segments (see "ipcs -m"). In that case, all
OS shared memory segments of a single IDS shared memory
segment are expected (by IDS) to be mapped adjacently. But if
the OS decides to do this in a different way (e.g. because a
shared lib segment "is in the way"), then things will not work out.
Hence such circumstances will limit the amount of memory that
IDS can use (as shared memory). If e.g. the shared libraries are
mapped into the middle of the address space, with the benevolent
idea of the OS to leave space for heap on one side and for stack
on the other, then this means that IDS is likely to use only half of
the available memory, on either side of "the barrier" of shared libs
segments. The other half will go pretty much unused, because IDS
makes little to no use of stack and heap.

So, there are many parameters that influence these things.
Therefore it is almost impossible to give any "fixed numbers" about
maximum possible memory use. I do not know of "an official IBM
web site or other place" giving such numbers. But with the above
explanation and some basic understanding of memory management ...
you can figure out the scenario on your system and the resulting
limitations.

Regards, Martin
--
Martin Fuerderer
IBM Informix Development Munich, Germany
Information Management

IBM Deutschland Research & Development GmbH
Chairman of the Supervisory Board: Martin Jetter
Board of Management: Dirk Wittkopp
Corporate Seat: Boeblingen, Germany
Reg.-Gericht: Amtsgericht Stuttgart, HRB 243294

ids-bounces@iiug.org wrote on 05/06/2010 12:01:14 PM:
> Dear all,
>
> I've googled until my fingers hurt on this subject and still can't get
it.
>
> In the new feature of IDS 10,it said we can specify shared memory size
greater
> than 4G. So in my understanding, in previous version the maximum memory
size
> is 4G. However I read some article show the maximum memory size for
32bit is
> 2G. Did I misunderstand what it said? what is the maximum you can
allocate for
> one instance (or all instances)? is it different between different
platform ?
> is there any official document in IBM web site or other place can help
me with
> this problem?
>
> thanks in advance,
> Fan
>
>
>
*******************************************************************************

> Forum Note: Use "Reply" to post a response in the discussion forum.
>

Messages In This Thread

• shared memory size limit on 32-bit vs 64 bit IDS
  FAN YANG -- Thursday, 6 May 2010, at 6:01 a.m.
  • Re: shared memory size limit on 32-bit vs 64 bit I
    Art Kagel -- Thursday, 6 May 2010, at 6:14 a.m.
  • Re: shared memory size limit on 32-bit vs 64 bit I
    Martin Fuerderer -- Thursday, 6 May 2010, at 8:12 a.m.
    • Re: shared memory size limit on 32-bit vs 64 bit I
      FAN YANG -- Friday, 7 May 2010, at 5:40 a.m.
      • Re: shared memory size limit on 32-bit vs 64 bit I
        Art Kagel -- Friday, 7 May 2010, at 6:39 a.m.
      • AW: shared memory size limit on 32-bit vs 64 bit I
        Andreas.KUTSCHE@spar.at -- Friday, 7 May 2010, at 6:59 a.m.

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