[sheepdog] [PATCH v2 1/3] sheep: make {register, unregister}_event thread safe

[Liu Yuan]

On Tue, Dec 17, 2013 at 04:33:58PM +0900, Hitoshi Mitake wrote:
> At Tue, 17 Dec 2013 15:11:30 +0800,
> Liu Yuan wrote:
> > >
> > > From the perspective of
> > > maintenance and debugging, I cannot agree with this patch. Adding a
> > > new workqueue for register/unregister in the main loop will save lots
> > > of our time in the future.
> > 
> > I think this approach will not work for async request handling, e.g,
> > 
> > +       uatomic_inc(&iocb->count);
> > +       req->iocb = iocb;
> > +       register_event(req->local_req_efd, local_req_async_handler, req);
> > +       submit_local_request(req);
> > 
> > I have to register the event before sumit_local_request().
> > 
> > If I don't and put registeration in a delegated thread, suppose that is called
> > after request is already handled. So I'll miss the notification that the request
> > is done and local_req_async_handler will never be called.
> > 
> > How do we handle above problem if we use your approach?
> > 
> > If we take performance into account, we can't do it in your approch either.
> > Suppose we will issue 1000 requests in a async mode, we can't affort to be trapped
> > to main thread to register event for every request.
> 
> I have an internal under construction change for parameterizing event
> loop. Current event loop mechanism has a limitation that we can have only one
> event loop per process. The change is for removing this limitation.
> 
> Making dedicated event loop which can be managed by worker threads would be
> helpful for your problem.

This is a more ratical change compared to my patch and I think it really needs
discusstion to do it.

> The change is originally intended for solving the problem that dog can use too
> many memory area in some case (e.g. checking large VDIs)
> 

I can't figure out why memory consumption is related to event loop. If you have
mutliple loop, it doesn't mean will cost less memory for threads.

Thanks
Yuan