Re: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait
Tue, 6 Feb 2001 19:20:21 +0530

>On Mon, Feb 05, 2001 at 08:01:45PM +0530, wrote:
>> >It's the very essence of readahead that we wake up the earlier buffers
>> >as soon as they become available, without waiting for the later ones
>> >to complete, so we _need_ this multiple completion concept.
>> I can understand this in principle, but when we have a single request
>> down to the device that actually fills in multiple buffers, do we get
>> notified (interrupted) by the device before all the data in that request
>> got transferred ?
>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.

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 ?
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

>> >Which is exactly why we have one kiobuf per higher-level buffer, and
>> >we chain together kiobufs when we need to for a long request, but we
>> >still get the independent completion notifiers.
>> As I mentioned above, the alternative is to have the i/o completion
>> linkage information within the wakeup structures instead. That way, it
>> doesn't matter to the lower level driver what higher level structure we
>> have above (maybe buffer heads, may be page cache structures, may be
>> kiobufs). We only chain together memory descriptors for the buffers
>> the io.
>You forgot IO failures: it is essential, once the IO completes, to
>know exactly which higher-level structures completed successfully and
>which did not. The low-level drivers have to have access to the
>independent completion notifications for this to work.
No, I didn't forget IO failures; just that I expect the wait structure
containing the wakeup function to be embedded in a cev structure that
contains a pointer to the wait_queue_head field in the higher level
structure. The rest is for the wakeup function to interpret (it can always
access the other fields in the higher level structure - just like
list_entry() does)

Later I realized that instead of having multiple wakeup functions queued on
the low level structures wait queue, its perhaps better to just sort of
turn the cev_wait structure upside down (entry on the lower level
structure's queue should link to the parent entries instead).

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