Re: 2.5.69, IDE TCQ can't be enabled

[Jeff Garzik]

On Mon, May 12, 2003 at 09:42:45PM +0200, Jens Axboe wrote:
> On Mon, May 12 2003, Jeff Garzik wrote:
> > Most Linux people with TCQ drives seem to have Hitachi (nee IBM)
> > ones AFAICS.  These do not have a service interrupt (or at least,
> > do not report such)
> 
> Nonsense, it supports the service interrupt just fine. It will just
> complain if you try to turn it off, iirc.

Weird.  Mine doesn't seem to assert it, nor does the identify page
indicate it's supported.  Maybe I have a broken drive firmware.

> > They do have the release interrupt.
> 
> Which we don't use. To be interesting, you need to speculatively turn on
> the dma engine for each command you want to start. If you don't do that,
> then it's faster just to poll for release/no-release at command start
> time.

That's an annoying thing about ATA TCQ:  the command _may_ execute
immediately, or may be queued (even when queue is empty).  At least
that's how I read the code and specs...

> I don't think the multiple pending _and_ active is that big a deal, and
> besides _everybody_ uses write back caching on IDE which makes TCQ for
> writes very uninteresting.
[...]
> I have to agree with Eric that the largest win is potentially not
> getting hit by the rotational latency all the time. I don't think you'll
> get much extra from actually having more than one active from the dma
> POV.

Yes and no.  I am coming from a driver-complexity perspective:
single-active is more annoying on the driver side.

In terms of drive performance, multiple active probably doesn't make
a huge difference.  In terms of reduction in host CPU usage, there
is a performance gain there with multiple active.

	Jeff