64bit - 32 bit AIX 5.1 considerations

[Tim Bruijnzeels]

Dear All,

I'm one of the administrators of an AIX 5.1 system (p660 6M 4CPUs). The
system is used as a multiuser system in a university environment. Currently
we are running the 32 bit kernel. Recently however one user ran into a
problem addressing files larger than 2 GB.
We made a special jfs filesystem with the large file flag set to true en I
set the limits for filesizes to unlimited in /etc/security/limits. Using dd
the user can indeed make a file > 2GB. Using the program that needs to
write to such a large file however the problem persists.

I have two questions:
1) Is this (as I suspect) a 32-bit kernel problem, and may it therefore be
solved by activating the 64 bit kernel?

2) Is there a risk that 32 bit applications that are now being used on the
system will no longer work under the 64bit kernel?

Other comments are also welcome of course.

Cheers,
Tim Bruijnzeels
System Administrator
University of Nijmegen

[Gary R. Hook]

Tim Bruijnzeels wrote:

> I'm one of the administrators of an AIX 5.1 system (p660 6M 4CPUs). The
> system is used as a multiuser system in a university environment. Currently
> we are running the 32 bit kernel. Recently however one user ran into a
> problem addressing files larger than 2 GB.
> We made a special jfs filesystem with the large file flag set to true en I
> set the limits for filesizes to unlimited in /etc/security/limits. Using dd
> the user can indeed make a file > 2GB. Using the program that needs to
> write to such a large file however the problem persists.

So far, so good.

> I have two questions:
> 1) Is this (as I suspect) a 32-bit kernel problem, and may it therefore be
> solved by activating the 64 bit kernel?

No. The problem is the application. It must be built with
large files enabled. (-D_LARGE_FILES)

> 2) Is there a risk that 32 bit applications that are now being used on the
> system will no longer work under the 64bit kernel?

They better not have any problems. Both types of apps should
function equivalently on both types of kernel.

--
Gary R. Hook / AIX PartnerWorld for Developers / These opinions are MINE
__________________________________________________ ______________________

[Andreas Schulze]

Tim Bruijnzeels <foutebingo@hotmail.com> wrote in message news:<bgr5a3$o13$1@odysseus.uci.kun.nl>...

> Dear All,
>
> I'm one of the administrators of an AIX 5.1 system (p660 6M 4CPUs). The
> system is used as a multiuser system in a university environment. Currently
> we are running the 32 bit kernel. Recently however one user ran into a
> problem addressing files larger than 2 GB.
> We made a special jfs filesystem with the large file flag set to true en I
> set the limits for filesizes to unlimited in /etc/security/limits. Using dd
> the user can indeed make a file > 2GB. Using the program that needs to
> write to such a large file however the problem persists.
>
> I have two questions:
> 1) Is this (as I suspect) a 32-bit kernel problem, and may it therefore be
> solved by activating the 64 bit kernel?
>
> 2) Is there a risk that 32 bit applications that are now being used on the
> system will no longer work under the 64bit kernel?
>
> Other comments are also welcome of course.
>
> Cheers,
> Tim Bruijnzeels
> System Administrator
> University of Nijmegen

Hallo Tim,

the 2GB size limit you describe might be caused by a POSIX compliance
of the program in question. E.g. you will hear of that problem quite
often in combination of backing up data with tar. Because of its POSIX
compliance you'll have to use backup/restore when saving/recovering
files bigger than 2 Gig. (Not to mention sparse files). It is
definitely not a 32-Bit Kernel problem but a problem of that
particular app.

Allegedly all software that has been written and compiled for 32-Bit
AIX will run unter the 64 Bit kernel. I did not come across any
program for that this was not true - yet. Of course there is no
guarantee, but in most cases it should work. However, as the 32-Bit
kernel has been around quite longer I'd recommend to stick to it as
long as you don't need the 64Bit kernel (e.g. as long as 96GB of RAM
is enough and your filesystems don't explode).

Regards,
Andreas

[Tim Bruijnzeels]

Thank you both for your comments.

So it definately looks like a problem on the app-level. Further inquiry
using google and the name of the app (molpro) came up with the same result.
There is a -largefile flag for the configure script. Next time I'll probe a
user like this for having followed installation instructions more
thouroughly.

The second question. I hear everywhere that 64 bits shouldn't be a problem.
But if one knows the 32bit kernel to perform well, an unneccessary
migration may be inprudent. If it works, don't touch, right?

But on the other hand I suspect that migrating to the 64 bit kernel might
very well be wise for us. The machine is primarily intended to facilitate
development and analysis (such as SPSS, SAS or the mentioned molpro
(theoretical chemistry)). I suspect that a wider bitrange provides faster
calculation rates for large numbers.
Is it true that a 64 bit kernel would give better performance? And if so,
what is the usual scale of this improvement, 1's, 10's 100's of percents?

Cheers,
Tim

[Gary R. Hook]

Tim Bruijnzeels wrote:

> I suspect that a wider bitrange provides faster
> calculation rates for large numbers.

Only if you're running a 64-bit application. A 32-bit app "sees" 32-bit
hardware no matter which kernel is running. 64-bit apps will be able
to move data in and out of the kernel faster because of the wider
registers (compare to the 32-bit kernel).

--
Gary R. Hook / AIX PartnerWorld for Developers / These opinions are MINE
__________________________________________________ ______________________

[Andreas Schulze]

Tim Bruijnzeels <foutebingo@hotmail.com> wrote in message news:<bgt1t9$adv$1@odysseus.uci.kun.nl>...

> Thank you both for your comments.
>
> So it definately looks like a problem on the app-level. Further inquiry
> using google and the name of the app (molpro) came up with the same result.
> There is a -largefile flag for the configure script. Next time I'll probe a
> user like this for having followed installation instructions more
> thouroughly.
>

Yes.

> The second question. I hear everywhere that 64 bits shouldn't be a problem.
> But if one knows the 32bit kernel to perform well, an unneccessary
> migration may be inprudent. If it works, don't touch, right?
>

Yes!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!

> But on the other hand I suspect that migrating to the 64 bit kernel might
> very well be wise for us. The machine is primarily intended to facilitate
> development and analysis (such as SPSS, SAS or the mentioned molpro
> (theoretical chemistry)). I suspect that a wider bitrange provides faster
> calculation rates for large numbers.
> Is it true that a 64 bit kernel would give better performance? And if so,
> what is the usual scale of this improvement, 1's, 10's 100's of percents?
>

You suspect, but you don't know ;-) Well, what I read between your
lines is that you have already made your decision to use the 64 Bit
kernel and don't really want to hear that the 32 Bit kernel might be
of good use. Thus I won't hold you back, that's far from me. And you
work at an university - that's the right place for any type of
experiments so go for it ;-). The 64Bit kernel has been installed
automatically on your server. Make sure that you have got the latest
ML and firmware installed. To give the 64 Bit a try by just rebooting
your pSeries do a

# ln -sf /usr/lib/boot/unix_64 /unix
# ln -sf /usr/lib/boot/unix_64 /usr/lib/boot/unix
# bosboot -ad /dev/ipldevice
# shutdown -Fr now

Btw - should you want to install your server from scratch with 64Bit
kernel enabled you are forced to use jfs2 also. (Again IMHO not as
mature als good old jfs). It might be a good idea not to use too much
new stuff at once. But again, that's up to you.

> Cheers,
> Tim

Enjoy!
Andreas