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

[Ben LaHaise]

On Tue, 6 Feb 2001, Ingo Molnar wrote:

>
> On Tue, 6 Feb 2001, Ben LaHaise wrote:
>
> > This small correction is the crux of the problem: if it blocks, it
> > takes away from the ability of the process to continue doing useful
> > work.  If it returns -EAGAIN, then that's okay, the io will be
> > resubmitted later when other disk io has completed.  But, it should be
> > possible to continue servicing network requests or user io while disk
> > io is underway.
>
> typical blocking point is waiting for page completion, not
> __wait_request(). But, this is really not an issue, NR_REQUESTS can be
> increased anytime. If NR_REQUESTS is large enough then think of it as the
> 'absolute upper limit of doing IO', and think of the blocking as 'the
> kernel pulling the brakes'.

=)  This is what I'm seeing: lots of processes waiting with wchan ==
__get_request_wait.  With async io and a database flushing lots of io
asynchronously spread out across the disk, the NR_REQUESTS limit is hit
very quickly.

> [overhead of 512-byte bhs in the raw IO code is an artificial problem of
> the raw IO code.]

True, and in the tests I've run, raw io is using 2KB blocks (same as the
database).

                -ben

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