Messages in this thread |  | | Date | Thu, 08 Feb 2001 15:11:06 +0100 | From | Martin Dalecki <> | Subject | Re: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait |
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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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