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SubjectRe: McVoy's Clusters (was Re: latest linus-2.5 BK broken)
Sandy Harris <> writes:

> [ I removed half a dozen cc's on this, and am just sending to the
> list. Do people actually want the cc's?]
> Larry McVoy wrote:
> > > Checkpointing buys three things. The ability to preempt jobs, the
> > > ability to migrate processes,

> For large multi-processor systems, it isn't clear that those matter
> much.

The systems that are built because there is no machine that can
run your compute intensive application fast enough they matter quite
a bit.

> What combination of resources and loads do you think preemption
> and migration are need for?

Good answers have already been given.
The problem domain I am looking at are compute clusters. The
solutions are useful elsewhere but in compute clusters they are
extremely valuable.

> > > and the ability to recover from failed nodes, (assuming the
> > > failed hardware didn't corrupt your jobs checkpoint).
> That matters, but it isn't entirely clear that it needs to be done
> in the kernel.

I agree, glibc would be fine, but it must be below the level of
the application. Generally it is a pretty onerous task to checkpoint
a random program. For a proof attempt to checkpoint your X desktop,
the infrastructure is there to do it.

Every application must be capable of checkpointing it for the cluster
batch scheduler to take advantage of it.

Example case.
You start job 1, a compute intensive application that runs for 4 days,
on 100 cpus. Your job is low priority.

In comes job2, a high priority job that runs for 4 hours and needs 256

job1 is preempted. With checkpoint support it can be saved and
restarted later. Without checkpointing support it is simply killed.

Migration is needed for failing hardware or to get low priority jobs
out of the way onto less capable nodes that are going unused.

Or to restart a job that failed on other hardware.

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