[ofa-general] Re: [PATCH] ib/ipoib: Reduce comparison size in data path

[Eli Cohen]

On Wed, 2008-01-02 at 10:16 -0800, Roland Dreier wrote:
> > In the majority of cases, if the neighbour will change, it will
>  > be reflected in the guid part of the GID (bytes 8-15). If the GID
>  > prefix will change as well (bytes 0-7) it will be because the master
>  > SM has changed, in which case we will get an SM change event resulting
>  > in all paths flushed.
> 
> Is it guaranteed that an active SM can't change a GID prefix?
I know opensm has a fixed, hard coded subnet prefix.

> Especially if we're using a GID at an index != 0?
I think ipoib uses only the GID from index 0, isn't it?
>   In other words, is
> this change definitely 100 percent safe?
It looks safe to me but I wanted to hear other opinions.
> 
> Also I assume this change is coming from performance tuning.  For
> patches like this it is always helpful to include hard data like "this
> gives a speedup of X on test Y on system Z."
> 
> Thanks...
Indeed I am working on performance and on the branch I am working on,
which is different than main branch, it does makes a slight difference.
I am trying to improve the throughput of small (up to 128 bytes) UDP
messages where I am CPU bound so everything counts. But I believe that
if it is correct than we should use it even if the improvement is not
outstanding.