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Subject: Re: [OMPI devel] Moving fragments in btl sm
From: Li-Ta Lo (ollie_at_[hidden])
Date: 2007-11-08 11:52:59

On Thu, 2007-11-08 at 13:38 +0100, Torje Henriksen wrote:
> Hi,
> I have a question that I shouldn't need to ask, but I'm
> kind of lost in the code.
> The btl sm component is using the circular buffers to write and read
> fragments (sending and receiving).
> In the write_to_head and read_from_tail I can only see pointers beeing set,
> no data being moved. So where does the actual data movement/copying take
> place? I'm thinking maybe a callback function existing somewhere :)
> Thank you for your help now and earlier.

You are right. The "real thing" happens at the
mca_btl_sm_component_progess(). The PML/BML will call btl_register()
to register callback function to be called when a frag is received.
In the event loop, the progress() function is called periodically to
check if there is any new frag arrived. It is complicated a little
bit by the fact that to transmit each "data" frag, there is a round
trip and two "frags" are exchanged. The send side sends the "data"
frag with header type SEND to the receiver. The receiver calls the
callback function the handle the frag and send back an ACK frag. Upon
receiving the ACK frag, the send side calls the des_cbfunc() to tell
the upper layer the the sending of this frag is completed.

BTW, it looks like it is still list append/remove in the PML/BML layer.
I don't know when/where the real "copying" happens.