Re: [PATCH V1 libibverbs 1/8] Add ibv_poll_cq_ex verb

Matan Barak (External)
2016-03-02T07:34:55Z

On 01/03/2016 19:24, Jason Gunthorpe wrote:
> On Tue, Mar 01, 2016 at 10:52:33AM +0200, Matan Barak (External) wrote:
>>> By my count it scores about a 2 on the Rusty API scale, which is
>>> pretty bad.
>>
>> This is like writing something that means nothing....
>
> If you don't agree with the points on the Rusty scale as what
> constitutes good API design then we really have no common ground on
> this point at all.
>

We can discuss actual points. That's how things can really progress.

>>> This series trade cache line efficiency for computation efficiency,
>>> and it isn't at all clear to me that is going to be a win. Better
>>> include some good benchmarks.
>>>
>>
>> WRONG. Which computational overhead are you talking about?
>
> I don't see the driver side posted, but obviously there must be
> indexing maths and possibly branching, depending on the design, that
> is overhead.
>

libmlx4 patches were posted on the mailing list awhile ago [1].
One of the patches there optimized ibv_poll_cq_ex for common cases with
almost zero overhead. Anyway, as I wrote before - this could be
delegated to a future hardware so I'm not sure it's relevant here.

>> The user-space driver writes one completion at a time, which (depending on
>> the user configuration) is probably less than a cache line. Why do you think
>> it's worse than how ibv_poll_cq behaves? The way the user-space driver
>> optimizes/dirties the cache line is unrelated to the API.
>
> I didn't say it was worse, I questioned that this should be the
> overarching goal when no data to support this has been posted and it
> doesn't even make inutitive sense that worrying about the size of
> *dirty cache lines* is even very important, (well, within limits).
>

We ran ib_send_bw (perftest package) when developing this - one time
using the regular ibv_poll_cq and another time with ibv_poll_cq_ex and
got identical results (the difference was tiny and not conclusive), but
I don't have the actual results right now. This was one of the common
cases the user-space driver optimized. We could re-run them and post
results if you want.

> Christoph may have some data, but I do wonder if his results are
> polluted by the current ibv_poll_cq driver implementations which do
> trigger RMW overheads.
>
>> Function per every completion fields permutation is uglier for sure (there
>> are currently 9 optional completion fields in this proposal, you could
>> easily do the math to calculate how many permutations we have).
>
> Why would you do every permutation?
>

Because user-space applications might want to get completion
time-stamping and size, but not the rest of the fields. They could
pretty much decide which data matters to them and get only that.

> Try a non-memcpy API design. Provide an opaque 'cursor' and functions
> to return certain data. This has more unconditional branches, but may
> avoid branching within the driver and certainly avoid memcpy's and
> pretty much all cache line dirtying. Sean had some results that
> were positive on this sort of direction.
>

Does the opaque pointer guarantees an aligned access? Who allocates the
space for the vendor's CQE? Any concrete example?
One of the problems here are that CQEs could be formatted as -
"if QP type is y, then copy the field z from o". Doing that this way may
result doing the same "if" multiple times. The current approach could
still avoid memcpy and write straight to the user's buffer.

> Justify this horrible API is *necessary* with something concrete, just
> not random hand waving and mumbling about cache lines. That means
> benchmark several options.
>
> Jason
>

Matan
[1] https://www.spinics.net/lists/linux-rdma/msg29837.html