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Subject: Re: [OMPI devel] initial SCTP BTL commit comments?
From: Brad Penoff (penoff_at_[hidden])
Date: 2007-11-13 15:41:15


On Nov 12, 2007 3:26 AM, Jeff Squyres <jsquyres_at_[hidden]> wrote:
> I have no objections to bringing this into the trunk, but I agree that
> an .ompi_ignore is probably a good idea at first.

I'll try to cook up a commit soon then!

> One question that I'd like to have answered is how OMPI decides
> whether to use the SCTP BTL or not. If there are SCTP stacks
> available by default in Linux and OS X -- but their performance may be
> sub-optimal and/or buggy, we may want to have the SCTP BTL only
> activated if the user explicitly asks for it. Open MPI is very
> concerned with "out of the box" behavior -- we need to ensure that
> "mpirun a.out" will "just work" on all of our supported platforms.

Just to make a few things explicit...

Things would only work out of the box on FreeBSD, and there the stack
is very good.

We have less experience with the Linux stack but hope the availability
of and SCTP BTL will help encourage its use by us and others. Now it
is a module by default (loaded with "modprobe sctp") but the actual
SCTP sockets extension API needs to be downloaded and installed
separately. The so-called lksctp-tools can be obtained here:
http://sourceforge.net/project/showfiles.php?group_id=26529

The OS X stack does not come by default but instead is a kernel extension:
http://sctp.fh-muenster.de/sctp-nke.html
I haven't yet started this testing but intend to soon. As of now
though, the supplied configure.m4 does not try to even build the
component on Mac OS X.

So in my opinion, things in the configure scripts should be fine the
way the are since only FreeBSD stack (which we have confidence in)
will try to work out of the box; the others require the user to
install things.

A question I had was with respect to what to set for the default value
of btl_sctp_exclusivity... I had wanted the exclusivity to be
"slightly less than TCP" so it was available but not the default. In
the code I set btl_sctp_exclusivity to this:
MCA_BTL_EXCLUSIVITY_LOW - 1
...however MCA_BTL_EXCLUSIVITY_LOW is defined as 0 and ompi_info says
that exclusivity must be >= 0... a -1 exclusivity doesn't seem to
break anything though... If two BTLs have the same exclusivity, what
is the tie-break? Alphabetic order?

>
> Will UBC setup regular MTT runs to test the SCTP stuff? :-)
>

We've only started playing with MTT so I'm sure we'll have plenty of
questions as we begin this process!

> More below.
>
>
> On Nov 10, 2007, at 9:25 PM, Brad Penoff wrote:
>
> >>> Currently, both the one-to-one and the one-to-many make use of the
> >>> event library offered by Open MPI. The callback functions for the
> >>> one-to-many style however are quite unique as multiple endpoints may
> >>> be interested in the events that poll returns. Currently we use
> >>> these
> >>> unique callback functions, but in the future the hope is to play
> >>> with
> >>> the potential benefits of a btl_progress function, particularly for
> >>> the one-to-many style.
> >>
> >> In my experience the event callbacks have a high overhead compared
> >> to a
> >> progress function, so I'd say thats definitely worth checking out.
> >
> > We noticed that poll is only called after a timer goes off while
> > btl_progress would be called with each iteration of opal_progress, so
> > noticing that along with you encouragement makes us want to check it
> > out even more.
>
>
> Be aware that based on discussions from the Paris meeting, some
> changes to libevent are coming (I really need to get this on a wiki
> page or something). Here's a quick summary:
>
> - We're waiting for a new release of libevent (or libev -- we'll see
> how it shakes out) that has lots of bug fixes and performance
> improvements as compared to the version we currently have in the OMPI
> tree. Based on some libevent mailing list traffic, this release may
> be in Dec 2007. We'll see what happens.
>
> - After we update libevent, we'll be making a policy change w.r.t.
> OMPI progress functions and timer callbacks: only software layers with
> actual devices will be allowed to register progress functions (in
> particular, the io and osd framework progress functions will be
> eliminated; see below). All other progress-requiring functions will
> have to use timers. This means that every time we call progress, we
> *only* call the stuff that needs to be polled as frequently as
> possible. We'll call the less-important progress stuff less
> frequently (e.g., ORTE OOB/RML).
>
> - We'll be changing our use of libevent to utilize the more scalable
> polling capabilities (such as epoll and friends). We don't use them
> right now because on all OS's that we currently care about (Linux, OS
> X, Solaris), mixing the scalable fd polling mechanism with pty's
> results in Very Very Bad Things. We'll special case where pty's are
> used and only use select/poll there, and then use epoll (etc.)
> elsewhere.
>
> - We'll also be changing our use of libevent to utilized timers
> properly.
>
> - ompi_request_t will be augmented to have a callback that, if non-
> NULL, will be invoked when the request is completed. This will allow
> removing the io and osd framework progress functions.
>
> - We may also add a high-performance clock framework in Open MPI -- a
> way of accessing high-resolution timers and clocks on the host (e.g.,
> on Intel chips, additional algorithms are necessary to normalize the
> per-chip clocks between sockets, especially if a process bounces
> between sockets -- unnecessary on AMD, PPC, and SPARC platforms).
> This could improve performance and precision of the libevent timers.
>
> - Finally, registering progress functions will take a new parameter: a
> file descriptor. If a file descriptor is provided and opal_progress()
> decides that it wants to block (specific mechanism TBD, but probably
> something similar to what other hybrid polling/blocking systems do:
> poll for a while, and if nothing "interesting" happens, block) *and*
> if all registered progress functions have valid fd's, then we'll block
> until either a timer expires or something "interesting" happens.
>

Thanks for the update on the things to come! We'll definitely keep an
eye on things as they arrive.

brad

> --
> Jeff Squyres
> Cisco Systems
>
>
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