Re: [PATCH 1/2] IB/isert: use unlikely macro in the fast path

[Leon Romanovsky <leonro@mellanox.com>]

On Wed, Aug 05, 2020 at 06:14:16PM +0300, Max Gurtovoy wrote:
>
> On 8/5/2020 4:16 PM, Leon Romanovsky wrote:
> > On Wed, Aug 05, 2020 at 03:12:30PM +0300, Max Gurtovoy wrote:
> > > Add performance optimization that might slightly improve small IO sizes
> > > benchmarks.
> > >
> > > Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
> > > ---
> > >   drivers/infiniband/ulp/isert/ib_isert.c | 4 ++--
> > >   1 file changed, 2 insertions(+), 2 deletions(-)
> > I find the expectation from "unlikely/likely" keywords to be overrated.
> >
> > When we introduced dissagregate post send verbs in rdma-core, we
> > benchmarked likely/unlikely and didn't find any significant difference
> > for code with and without such keywords.
> >
> > Thanks
>
> Leon,
>
> We are using these small optimizations in all our ULPs and we saw benefit in
> large scale and high loads (we did the same in NVMf/RDMA).
>
> These kind of optimizations might not be seen immediately but are
> accumulated.
>
> I don't know why do you compare user-space benchmarks to storage drivers.

Why not? It produces same asm code and both have same performance
characteristic.

>
> Can you please review the code ?

There is nothing to review here, the patch is straightforward, I just
don't believe in it.

>
> Sagi,
>
> Can you send your comments as well ?
>
>