Re: [net-next][PATCH v2 11/13] RDS: IB: add Fastreg MR (FRMR) detection support

[santosh.shilimkar@xxxxxxxxxx]

On 2/28/16 1:08 AM, Christoph Hellwig wrote:
> On Sat, Feb 27, 2016 at 06:19:48PM -0800, Santosh Shilimkar wrote:
> > Discovere Fast Memmory Registration support using IB device
> > IB_DEVICE_MEM_MGT_EXTENSIONS. Certain HCA might support just FRMR
> > or FMR or both FMR and FRWR. In case both mr type are supported,
> > default FMR is used.
> >
> > Default MR is still kept as FMR against what everyone else
> > is following. Default will be changed to FRMR once the
> > RDS performance with FRMR is comparable with FMR. The
> > work is in progress for the same.
> >
> > Signed-off-by: Santosh Shilimkar <ssantosh@xxxxxxxxxx>
> > Signed-off-by: Santosh Shilimkar <santosh.shilimkar@xxxxxxxxxx>
> > ---
> > v2: Dropped the module parameter as suggested by David Miller
>
> This means we only use the safer method if the HCA doesn't support
> the other one.  All other RDMA ULP that support both methods have
> a module_param so the veto from Dave is a bit unfortunate.  Anyway,
> let's get the code in for now and figure out what to use later.
Indeed. It wasn't really deal breaker for RDS so I agreed to drop it.

> Just curious:  where / how do you see worse peformance using FRs?
I wouldn't call it worse but its not comparable. Use case
is multi-threaded RDS RDMA perf test(s). Hopefully the follow
up series which am working on should minimise the gap. I am
leaving the details for later, but one of the main issue I
saw was contention on driver post_send() lock from send, MR reg
and MR invalidation.

Regards,
Santosh