Re: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait /notify + callback chains

[Jens Axboe]

On Tue, Feb 06 2001, [e-mail] wrote:
> >It depends on the device driver.  Different controllers will have
> >different maximum transfer size.  For IDE, for example, we get wakeups
> >all over the place.  For SCSI, it depends on how many scatter-gather
> >entries the driver can push into a single on-the-wire request.  Exceed
> >that limit and the driver is forced to open a new scsi mailbox, and
> >you get independent completion signals for each such chunk.

SCSI does not build a request bigger than the low level driver
can handle. If you exceed the scatter count in a single request,
you just stop and fire of that request, later on restarting I/O
on the remainder.

> I see. I remember Jens Axboe mentioning something like this with IDE.
> So, in this case, you want every such chunk to check if its completed
> filling up a buffer and then trigger a wakeup on that ?

Yes. Which is why dealing with buffer heads are so nice in this
regard, you never have problems with ending I/O on a single "piece".

> But, does this also mean that in such a case combining requests beyond this
> limit doesn't really help ? (Reordering requests to get contiguity would
> help of course in terms of seek times, I guess, but not merging beyond this
> limit)

There's a slight benefit in building bigger requests than the driver
can handle, in that you can have more I/O pending on the queue. It's
not worth spending too much time on though.

-- 
Jens Axboe