Messages in this thread | | | Date | Tue, 13 May 2003 08:40:59 +0200 | From | Jens Axboe <> | Subject | Re: 2.5.69, IDE TCQ can't be enabled |
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On Mon, May 12 2003, Jeff Garzik wrote: > 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.
Then the linux code won't work on it, have you tried? I've tried a lot of different IBM models, they all do service interrupts just fine.
> > > 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...
That's correct, you can use the release interrupt to get around that...
> > 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.
It should make a non-neglible difference in smart positioning in the drive, some things just cannot be done in software for this stuff.
-- Jens Axboe
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