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

["ira.weiny" <ira.weiny-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

On Mon, Mar 23, 2015 at 11:17:49AM -0600, Jason Gunthorpe wrote:
> 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.

I concur with Jason's assessment.

As Yann asked before:

What hardware have you tested this on?  Do you have any performance
measurements?  Or do you have a reproducer for some of the allocation issues
which have been seen?

I can't comment on how this may affect Mellanox Hardware but it seems like it
will work fine with Qib hardware.

Ira


> 
> Jason
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