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SubjectRe: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait
Linus Torvalds wrote:
>
> On Tue, 6 Feb 2001, Ben LaHaise wrote:
> >
> > On Tue, 6 Feb 2001, Stephen C. Tweedie wrote:
> >
> > > The whole point of the post was that it is merging, not splitting,
> > > which is troublesome. How are you going to merge requests without
> > > having chains of scatter-gather entities each with their own
> > > completion callbacks?
> >
> > Let me just emphasize what Stephen is pointing out: if requests are
> > properly merged at higher layers, then merging is neither required nor
> > desired.
>
> I will claim that you CANNOT merge at higher levels and get good
> performance.
>
> Sure, you can do read-ahead, and try to get big merges that way at a high
> level. Good for you.
>
> But you'll have a bitch of a time trying to merge multiple
> threads/processes reading from the same area on disk at roughly the same
> time. Your higher levels won't even _know_ that there is merging to be
> done until the IO requests hit the wall in waiting for the disk.

Merging is a hardware tighted optimization, so it should happen, there
we you
really have full "knowlendge" and controll of the hardware -> namely the
device driver.

> Qutie frankly, this whole discussion sounds worthless. We have solved this
> problem already: it's called a "buffer head". Deceptively simple at higher
> levels, and lower levels can easily merge them together into chains and do
> fancy scatter-gather structures of them that can be dynamically extended
> at any time.
>
> The buffer heads together with "struct request" do a hell of a lot more
> than just a simple scatter-gather: it's able to create ordered lists of
> independent sg-events, together with full call-backs etc. They are
> low-cost, fairly efficient, and they have worked beautifully for years.
>
> The fact that kiobufs can't be made to do the same thing is somebody elses
> problem. I _know_ that merging has to happen late, and if others are
> hitting their heads against this issue until they turn silly, then that's
> their problem. You'll eventually learn, or you'll hit your heads into a
> pulp.

Amen.

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