Re: [PATCH] ib_ipoib: Scatter-Gather support in connected mode

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Sun, Mar 22, 2015 at 11:21:50AM +0200, Yuval Shaia wrote:
> On Sun, Mar 15, 2015 at 05:16:16PM +0200, Yuval Shaia wrote:
> > Hi,
> > I didn't got any further comments on this one.
> > Any idea why SG in CM is un-welcome?
> By mistake I sent a private mail only.
> Cc: Roland Dreier <roland-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> Cc: Sean Hefty <sean.hefty-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> Cc: Hal Rosenstock <hal.rosenstock-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> 
> Your advice would be very appreciated.

I haven't looked in detail at the patch, but in principle, using S/G
when ever possible should be the default, even if this creates a
performance regression.

It is well known that high order allocations are problematic in Linux
and should be avoided, and I also have seen systems blow up because of
high order IPoIB allocations.

That said, there may be cases where S/G is not possible, you should
try and get Mellanox to comment if all their offloads work on all
their cards when S/G is used. Work may be required to resolve any of
these constraints. I'd like to belive there is some reason why we've
been doing high order allocations for so many years.

FWIW, I would probably choose to default S/G over any other offload
acceleration.

Jason
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