device plugging

device plugging

[gopi krishna]

Why do we need a dummy req for plugging.
As i understood only thing plugging does is to, on arrival of new req if
the dev queue is empty, puts a dummy req on the queue, and schedules the
unplug routine on tq_disk, which on being scheduled calls the strategy
routine.
So we can as well put the new req on the queue without dummy req.

If i'm incorrect please explain what's exact process and the reason

Please cc the response to mgopi@indiainfo.com as i'm not subscribed.

thanks
----------------------------------------
http://mail.indiainfo.com
First you had 10MB of free mail space.
Now you can send mails in your own language !!!

Re: device plugging

[Jens Axboe]

On Sat, Jul 07 2001, gopi krishna wrote:
> Why do we need a dummy req for plugging.
> As i understood only thing plugging does is to, on arrival of new req if
> the dev queue is empty, puts a dummy req on the queue, and schedules the
> unplug routine on tq_disk, which on being scheduled calls the strategy
> routine.

You are reading 2.2 sources, maybe try the 2.4 sources as they are
easier to follow in this regard IMHO.

What happens is that we assign dev->current_request &dev->plug, so that
the front request is recognizable as the 'plug' and not a valid request.
You seem to completely misunderstand plugging -- if we just put the new
request on the queue and scheduled tq_disk, then there would be no
plugging going on at all. What we do instead is add this specific 'plug
request' and queue lots of stuff behind that. Not until someone needs
the data on the queue is the 'plug request' removed and the request_fn
run. This happens for instance if _someone else_ calls tq_disk,
typically __wait_on_buffer.

> So we can as well put the new req on the queue without dummy req.

Wrong

> If i'm incorrect please explain what's exact process and the reason

See?

-- 
Jens Axboe

Re: device plugging

[gopi krishna]

Hello,
Can some one provide online pointers for the description and features
of linux 2.4 kernel.
The rubini 2nd edn is not avlbl in india and most of the books
still concentrate on 2.2.
Plz cc the responses to mgopi@indiainfo.com as i am no subscribed to the 
group
----------------------------------------
http://mail.indiainfo.com
First you had 10MB of free mail space.
Now you can send mails in your own language !!!