Messages in this thread |  | | From | bsuparna@in ... | Date | Thu, 1 Feb 2001 20:14:58 +0530 | Subject | Re: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait /notify + callback chains |
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>Hi, > >On Thu, Feb 01, 2001 at 10:25:22AM +0530, bsuparna@in.ibm.com wrote: >> >> >We _do_ need the ability to stack completion events, but as far as the >> >kiobuf work goes, my current thoughts are to do that by stacking >> >lightweight "clone" kiobufs. >> >> Would that work with stackable filesystems ? > >Only if the filesystems were using VFS interfaces which used kiobufs. >Right now, the only filesystem using kiobufs is XFS, and it only >passes them down to the block device layer, not to other filesystems.
That would require the vfs interfaces themselves (address space readpage/writepage ops) to take kiobufs as arguments, instead of struct page * . That's not the case right now, is it ? A filter filesystem would be layered over XFS to take this example. So right now a filter filesystem only sees the struct page * and passes this along. Any completion event stacking has to be applied with reference to this.
>> Being able to track the children of a kiobuf would help with I/O >> cancellation (e.g. to pull sub-ios off their request queues if I/O >> cancellation for the parent kiobuf was issued). Not essential, I guess, in >> general, but useful in some situations. > >What exactly is the justification for IO cancellation? It really >upsets the normal flow of control through the IO stack to have >voluntary cancellation semantics.
One reason that I saw is that if the results of an i/o are no longer required due to some condition (e.g. aio cancellation situations, or if the process that issued the i/o gets killed), then this avoids the unnecessary disk i/o, if the request hadn't been scheduled as yet.
Too remote a requirement ? If the capability/support doesn't exist at the driver level I guess its difficult.
--Stephen
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