Re: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait

From: Ben LaHaise (
Date: Tue Feb 06 2001 - 14:11:23 EST

On Tue, 6 Feb 2001, Ingo Molnar wrote:

> On Tue, 6 Feb 2001, Ben LaHaise wrote:
> > - reduce the overhead in submitting block ios, especially for
> > large ios. Look at the %CPU usages differences between 512 byte
> > blocks and 4KB blocks, this can be better.
> my system is already submitting 4KB bhs. If anyone's raw-IO setup submits
> 512 byte bhs thats a problem of the raw IO code ...
> > - make asynchronous io possible in the block layer. This is
> > impossible with the current ll_rw_block scheme and io request
> > plugging.
> why is it impossible?

s/impossible/unpleasant/. ll_rw_blk blocks; it should be possible to have
a non blocking variant that does all of the setup in the caller's context.
Yes, I know that we can do it with a kernel thread, but that isn't as
clean and it significantly penalises small ios (hint: databases issue
*lots* of small random ios and a good chunk of large ios).

> > You mentioned non-spindle base io devices in your last message. Take
> > something like a big RAM disk. Now compare kiobuf base io to buffer
> > head based io. Tell me which one is going to perform better.
> roughly equal performance when using 4K bhs. And a hell of a lot more
> complex and volatile code in the kiobuf case.

I'm willing to benchmark you on this.


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