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,
