Is a configure-time test good enough?  For example, are all Linuxes the same in this regard.  That is if you built OMPI on RH and it configured in the new SysV SM will those bits actually run on other Linux systems correctly?  I think Jeff had hinted to this similarly when suggesting this may need to be a runtime test. 


Samuel K. Gutierrez wrote:
Hi All,

New configure-time test added - thanks for the suggestion, Jeff.  Update and give it a whirl.

Ethan - could you please try again?  This time, I'm hoping sysv support will be disabled ;-).


Samuel K. Gutierrez
Los Alamos National Laboratory

On May 3, 2010, at 9:18 AM, Samuel K. Gutierrez wrote:

Hi Jeff,

Sounds like a plan :-).


Samuel K. Gutierrez
Los Alamos National Laboratory

On May 3, 2010, at 9:12 AM, Jeff Squyres wrote:

It might well be that you need a configure test to determine whether this behavior occurs or not.  Heck, it may even need to be a run-time test!  Hrm.

Write a small C program that does something like the following (this is off the top of my head):

fork a child
child goes to sleep immediately
sysv alloc a segment
attach to it
ipc rm it
parent wakes up child
child tries to attach to segment

If that succeeds, then all is good.  If not, then don't use this stuff.

On May 3, 2010, at 10:55 AM, Samuel K. Gutierrez wrote:

Hi all,

Does anyone know of a relatively portable solution for querying a
given system for the shmctl behavior that I am relying on, or is this
going to be a nightmare?  Because, if I am reading this thread
correctly, the presence of shmget and Linux is not sufficient for
determining an adequate level of sysv support.


Samuel K. Gutierrez
Los Alamos National Laboratory

On May 2, 2010, at 7:48 AM, N.M. Maclaren wrote:

On May 2 2010, Ashley Pittman wrote:
On 2 May 2010, at 04:03, Samuel K. Gutierrez wrote:

As to performance there should be no difference in use between sys-
V shared memory and file-backed shared memory, the instructions
issued and the MMU flags for the page should both be the same so
the performance should be identical.

Not necessarily, and possibly not so even for far-future Linuces.
On at least one system I used, the poxious kernel wrote the complete
file to disk before returning - all right, it did that for System V
shared memory, too, just to a 'hidden' file!  But, if I recall, on
another it did that only for file-backed shared memory - however, it's
a decade ago now and I may be misremembering.

Of course, that's a serious issue mainly for large segments.  I was
using multi-GB ones.  I don't know how big the ones you need are.

The one area you do need to keep an eye on for performance is on
numa machines where it's important which process on a node touches
each page first, you can end up using different areas (pages, not
regions) for communicating in different directions between the same
pair of processes. I don't believe this is any different to mmap
backed shared memory though.

On some systems it may be, but in bizarre, inconsistent, undocumented
and unpredictable ways :-(  Also, there are usually several system
sometimes user) configuration options that change the behaviour, so
have to allow for that.  My experience of trying to use those is that
different uses have incompatible requirements, and most of the
configuration parameters apply to ALL uses!

In my view, the configuration variability is the number one nightmare
for trying to write portable code that uses any form of shared memory.
ARMCI seem to agree.

Because of this, sysv support may be limited to Linux systems -
that is,
until we can get a better sense of which systems provide the shmctl
IPC_RMID behavior that I am relying on.

And, I suggest, whether they have an evil gotcha on one of the areas
Ashley Pittman noted.

Nick Maclaren.

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