Re: [PATCH V4 libibverbs 2/7] Add member functions to poll an extended CQ

Doug Ledford
2016-05-30T03:39:35Z

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On 5/29/2016 9:35 PM, Jason Gunthorpe wrote:
> On Sun, May 29, 2016 at 07:47:54PM -0400, Doug Ledford wrote:
>>> You should really read the whole prior discussion
>>
>> If there's something in particular you'd like to point out, feel free.
>
> You can't ignore the mailing list and expect the rest of us to carry
> you along when you finally wake up..

Who said I ignored anything? I didn't get your reference and needed you
to be more explicit. You don't have to be a dick about it.

>> It doesn't have to be refs. If I recall correctly, the libmlx4
>> driver has to clear each cqe by changing the ownership status back
>> to the hardware or something like that. The release could just as
>> easily be setting that ownership on the cqe.
>
> It also has to notify the hardware of the new ring head pointer which
> becomes more expensive with refs, especially on a lockable CQ.

It's no more expensive under what I just outlined.

>> The original bitmask version included a copy to an interim, variable
>> struct. The idea here is that the cqe is an opaque pointer directly
>> to the hardware cqe. There would be no interim copies, just
>> retrieving the desired data directly from the cqe. Maybe I should
>> have called it a hcqe to be more clear.
>
> Unless I'm mistaken, something much more more complex than just a
> simple offsets is required to handle the usual hcqe format used by our
> various drivers.

That's a valid point of discussion.

> I agree if you could describe the typical hcqe extractor in a format
> like offsets/mask/shift for all supported drivers then that might be
> better (unclear if an unaligned load/mask/shift is more expensive than
> a branch). I believe I already explained that possibility earlier in
> the thread, and went further to suggest doing a direct HW optimized
> DIRECT option like libfabric has.

And that's what I want to explore.

> However, getting this wrong limits the future hardware we can support,
> which is scary, and the branch option clearly doesn't have that
> problem.
>
> So the performance win would have to be substantial, IMHO.

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