Open MPI logo

Open MPI Development Mailing List Archives

  |   Home   |   Support   |   FAQ   |   all Development mailing list

Subject: Re: [OMPI devel] RFC: changes to modex
From: Gleb Natapov (glebn_at_[hidden])
Date: 2008-04-03 09:18:28

On Thu, Apr 03, 2008 at 07:05:28AM -0600, Ralph H Castain wrote:
> Hmmmm...since I have no control nor involvement in what gets sent, perhaps I
> can be a disinterested third party. ;-)
> Could you perhaps explain this comment:
> > BTW I looked at how we do modex now on the trunk. For OOB case more
> > than half the data we send for each proc is garbage.
> What "garbage" are you referring to? I am working to remove the stuff
> inserted by proc.c - mostly hostname, hopefully arch, etc. If you are
> running a "debug" version, there will also be type descriptors for each
> entry, but those are eliminated for optimized builds.
> So are you referring to other things?
I am talking about openib part of the modex. The "garbage" I am
referring to is this:

This is the structure that is sent by modex for each openib BTL. We send entire
structure by copying it into a message.
struct mca_btl_openib_port_info {
    uint32_t mtu;
    uint8_t padding[4];
    uint64_t subnet_id;
    uint16_t lid; /* used only in xrc */
    uint16_t apm_lid; /* the lid is used for APM to
                         different port */
   char *cpclist;

The sizeof() the struct is 32 byte, but how much useful info it actually
mtu - should really be uint8 since this is encoded value (1,2,3,4)
padding - is garbage.
sibnet_id - is ok
lid - should be sent only for XRC case
apm_lid - should be sent only if apm is enabled
cpclist - is pure garbage and should not be in this struct at all.

So we send 32 bytes with only 9 bytes of useful info (for non XRC case
without APM enabled).