Re: [PATCH linux-2.6-block:master] blk: reimplement elevator switch

[Jens Axboe]

On Thu, Oct 20 2005, Tejun Heo wrote:
> On Thu, Oct 20, 2005 at 02:25:05PM +0200, Jens Axboe wrote:
> > On Wed, Oct 19 2005, Tejun Heo wrote:
> > >  Hello, Jens.
> > > 
> > >  This patch reimplements elevator switch.  This patch assumes generic
> > > dispatch queue patchset is applied.
> > > 
> > >  * Each request is tagged with REQ_ELVPRIV flag if it has its elevator
> > >    private data set.
> > >  * Requests which doesn't have REQ_ELVPRIV flag set never enter
> > >    iosched.  They are always directly back inserted to dispatch queue.
> > >    Of course, elevator_put_req_fn is called only for requests which
> > >    have its REQ_ELVPRIV set.
> > >  * Request queue maintains the current number of requests which have
> > >    its elevator data set (elevator_set_req_fn called) in
> > >    q->rq->elvpriv.
> > >  * If a request queue has QUEUE_FLAG_BYPASS set, elevator private data
> > >    is not allocated for new requests.
> > > 
> > >  To switch to another iosched, we set QUEUE_FLAG_BYPASS and wait until
> > > elvpriv goes to zero; then, we attach the new iosched and clears
> > > QUEUE_FLAG_BYPASS.  New implementation is much simpler and main code
> > > paths are less cluttered, IMHO.
> > 
> > Wonderful! Applied as-is, I didn't make any changes to this one. I agree
> > it's much cleaner than the previous approach, both in the code and in
> > killing the request_queue and request_list members.
> > 
> > I'm going to make a little few tweaks:
> > 
> > - The naming, QUEUE_FLAG_BYPASS isn't really clear. I don't know what
> >   this means without looking at specific parts of the code. Testing of
> 
>  It means to bypass ioscheds and go directly into dispatch queue.

Certainly :-) I renamed it to QUEUE_FLAG_ELVSWITCH for now, if another
use comes of this we can name it something appropriately generic then.

> >   same flag in various locations would also be preferred instead of
> >   passing priv around and cluttering the function parameters, however we
> >   should split the queue flags a little for this. Basically into an
> >   atomic and non-atomic part. So I'll leave that alone for now.
> 
>  Hmmm...

There's room for optimization there, lots of places we check queue flags
(and set them) inside the queue lock, we don't need to use the bit
operations for those. But we cannot safely mix them either.

> > - The msleep(100) seems a little too slow. With the switching being more
> >   efficient now, in 100msecs we can complete lots of requests.
> 
>  Yeap, agreed.

Cool

-- 
Jens Axboe

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