> -----Original Message----- > From: Troy Benjegerdes [mailto:[EMAIL PROTECTED] > Sent: Monday, March 14, 2005 5:06 PM > To: Caitlin Bestler > Cc: openib-general@openib.org > Subject: Re: [openib-general] Getting rid of pinned memory requirement > > > > > The key is that the entire operation either has to be fast > > enough so that no connection or application session layer > > time-outs occur, or an end-to-end agreement to suspend the > > connetion is a requirement. The first option seems more > > plausible to me, the second essentially > > reuqires extending the CM protocol. That's a tall order even for > > InfiniBand, and it's even worse for iWARP where the CM > > functionality typically ends when the connection is established. > > I'll buy the good network design argument. > > I suppose if the kernel wants to revoke a card's pinned > memory, we should be able to guarantee that it gets new > pinned memory within a bounded time. What sort of timing do > we need? Milliseconds? > Microseconds? > > In the case of iWarp, isn't this just TCP underneath? If so, > can't we just drop any packets in the pipe on the floor and > let them get retransmitted? (I suppose the same argument goes > for infiniband.. > what sort of a time window do we have for retransmission?) > > What are the limits on end-to-end flow control in IB and iWarp? >
>From the RDMA Provider's perspective, the short answer is "quick enough so that I don't have to do anything heroic to keep the connection alive." With TCP you also have to add "and healthy". If you've ever had a long download that got effectively stalled by a burst of noise and you just hit the 'reload' button on your browser then you know what I'm talking about. But in transport neutral terms I would think that one RTT is definitely safe -- that much data could have been dropped by one switch failure or one nasty spike in inbound noise. > > > > Yes, there are limits on how much memory you can mlock, or even > > allocate. Applications are required to reqister memory precisely > > because the required guarantess are not there by default. > Eliminating > > those guarantees *is* effectively rewriting every RDMA application > > without even letting them know. > > Some of this argument is a policy issue, which I would argue > shouldn't be hard-coded in the code or in the network hardware. > > At least in my view, the guarantees are only there to make > applications go fast. We are getting low latency and high > performance with infiniband by making memory registration go > really really slow. If, to make big HPC simulation > applications work, we wind up doing memcpy() to put the data > into a registered buffer because we can't register half of > physical memory, the application isn't going very fast. > What you are looking for is a distinction between registering memory to *enable* the RNIC to optimize local access and registering memory to enable its being advertised to the remote end. Early implementations of RDMA, both IB and iWARP, have not distinquished between the two. But theoretically *applications* do not need memory regions that are not enabled for remote access to be pinned. That is an RNIC requirement that could evolve. But applications themselves *do* need remotely accessible memory regions, portions of which they intend to advertise with RKeys, to be truly available (i.e., pinned). You are also making a policy assumption that an application that actually needs half of physical memory should be using paged memory. Memory is cheap, and if performance is critical why should this memory be swapped out to disk? Is the limitation on not being able to register half of physical memory based upon some assumption that swapping is a requirement? Or is it a limitation in the memory region size? If it's the latter, you need to get the OS to support larger page sizes. _______________________________________________ openib-general mailing list openib-general@openib.org http://openib.org/mailman/listinfo/openib-general To unsubscribe, please visit http://openib.org/mailman/listinfo/openib-general
