Re: [PATCH 0/2] abstract napi tracking strategy

[Olivier Langlois <olivier@trillion01.com>]

On Tue, 2024-08-13 at 12:33 -0600, Jens Axboe wrote:
> On 8/13/24 10:44 AM, Olivier Langlois wrote:
> > the actual napi tracking strategy is inducing a non-negligeable
> > overhead.
> > Everytime a multishot poll is triggered or any poll armed, if the
> > napi is
> > enabled on the ring a lookup is performed to either add a new napi
> > id into
> > the napi_list or its timeout value is updated.
> > 
> > For many scenarios, this is overkill as the napi id list will be
> > pretty
> > much static most of the time. To address this common scenario, a
> > new
> > abstraction has been created following the common Linux kernel
> > idiom of
> > creating an abstract interface with a struct filled with function
> > pointers.
> > 
> > Creating an alternate napi tracking strategy is therefore made in 2
> > phases.
> > 
> > 1. Introduce the io_napi_tracking_ops interface
> > 2. Implement a static napi tracking by defining a new
> > io_napi_tracking_ops
> 
> I don't think we should create ops for this, unless there's a strict
> need to do so. Indirect function calls aren't cheap, and the CPU side
> mitigations for security issues made them worse.
> 
> You're not wrong that ops is not an uncommon idiom in the kernel, but
> it's a lot less prevalent as a solution than it used to. Exactly
> because
> of the above reasons.
> 
ok. Do you have a reference explaining this?
and what type of construct would you use instead?

AFAIK, a big performance killer is the branch mispredictions coming
from big switch/case or if/else if/else blocks and it was precisely the
reason why you removed the big switch/case io_uring was having with
function pointers in io_issue_def...

I consumme an enormous amount of programming learning material daily
and this is the first time that I am hearing this.

If there was a performance concern about this type of construct and
considering that my main programming language is C++, I am bit
surprised that I have not seen anything about some problems with C++
vtbls...

but oh well, I am learning new stuff everyday, so please share the
references you have about the topic so that I can perfect my knowledge.

thank you,