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Subject: Re: [OMPI devel] Threaded progress for CPCs
From: Gleb Natapov (glebn_at_[hidden])
Date: 2008-05-19 15:12:19


On Mon, May 19, 2008 at 01:52:22PM -0500, Jon Mason wrote:
> On Mon, May 19, 2008 at 05:17:57PM +0300, Gleb Natapov wrote:
> > On Mon, May 19, 2008 at 05:08:17PM +0300, Pavel Shamis (Pasha) wrote:
> > > >> 5. ...?
> > > >>
> > > > What about moving posting of receive buffers into main thread. With
> > > > SRQ it is easy: don't post anything in CPC thread. Main thread will
> > > > prepost buffers automatically after first fragment received on the
> > > > endpoint (in btl_openib_handle_incoming()).
> > > It still doesn't guaranty that we will not see RNR (as I understand we
> > > trying to resolve this problem for iwarp?!)
> > >
> > I don't think that iwarp has SRQ at all. And if it has then it should
>
> While Chelsio does not currently have an adapter that has SRQs, there are
> some other iWARP vendors that do have them.
>
> > have HW flow control for it too. I don't see what advantage SRQ without
> > flow control can provide over PPRQ.
>
> Technically, this is not flow control, it is a retransmit. iWARP can use
> the HW TCP stack to retransmit, but it will not have the "retransmit
> forever" ability that setting rnr_retry to 7 has for IB.
For how long will it try to retransmit before dropping connection.

>
> > > So this solution will cost 1 buffer on each srq ... sounds acceptable
> > > for me. But I don't see too much
> > > difference compared to #1, as I understand we anyway will be need the
> > > pipe for communication with main thread.
> > > so why don't use #1 ?
> > What communication? No communication at all. Just don't prepost buffers
> > to SRQ during connection establishment. Problem solved (only for SRQ of
> > cause).
>
> iWARP needs preposted recv buffers (or it will drop the connection). So
> this isn't a good option.
I was talking about SRQ only. You said above that iwarp does retransmit for SRQ.
openib BTL relies on HW retransmit when using SRQ, so if iwarp doesn't do it
reliably enough it can not be used with SRQ anyway.

--
			Gleb.