IDS Forum

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

Posted By: Art Kagel
Date: Friday, 7 May 2010, at 6:39 a.m.

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

Some 32 bit architectures use a 64 bit address space internally so they can
have more than 4GB of memory addressable by the OS. However, 32bit
applications will still only have a 32bit address space, it's just that the
starting physical address can be greater than 4GB. IB that's what the notes
are referring to. To be absolutely clear, no 32 bit instance of Informix
can address more than 4GB of memory and on most platforms they are
practically limited by the stack, heap, and shared library attach addresses
to something closed to 3.2GB. On some platforms like AIX that can be as log
as 2.2GB.

Art

Art S. Kagel
Advanced DataTools (www.advancedatatools.com)
IIUG Board of Directors (art@iiug.org)

See you at the 2010 IIUG Informix Conference
April 25-28, 2010
Overland Park (Kansas City), KS
www.iiug.org/conf

Disclaimer: Please keep in mind that my own opinions are my own opinions and
do not reflect on my employer, Advanced DataTools, the IIUG, nor any other
organization with which I am associated either explicitly, implicitly, or by
inference. Neither do those opinions reflect those of other individuals
affiliated with any entity with which I am affiliated nor those of the
entities themselves.

On Fri, May 7, 2010 at 5:40 AM, FAN YANG <fania.yang@gmail.com> wrote:

> hi, Martin
>
> thanks for your reply
>
> However,I read about the feature nots in the machine nots for IDS 9.40 FC1.
> The larger memory addressability (LMA) provides the ability to support
> system
> configurations with greater than 4GB of RAM. And it shows that the maximum
> size of any shared memory segment is 4278190080 bytes. In the other side,
> the
> new feature in IDS 10, it also shows it can support shared memory size
> greater
> than 4G.So now I am confused and I really can't figure out what they talk
> about.
>
> Fan
>
> ***********************************************************************
>
> 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.
>
>

--000e0cd6085459f1f10485fead16

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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