Re: Resource Management on General-Purpose Systems

[Jonathan S. Shapiro]

On Thu, 2007-07-12 at 17:53 +0100, Sam Mason wrote:
> > OTOH, disk I/O scheduling is considered a hard problem.
> 
> Guaranteeing bandwidth/latency to specific processes seems hard to me.

Actually, guaranteeing bandwidth/latency isn't particularly hard. What
is hard is preserving any sort of sensible behavior for the rest of the
system at the same time.

The problem with disks is not high latency per se -- we have very good
models for seek latency -- as high *variance*. The dominant latency is
actually rotational these days, and the problem is that the rotational
delay is not terribly predictable. Problem is that this can raise or
lower the time required for the operation by almost a factor of two, so
you get forced into very conservative resource allocation here.
-- 
Jonathan S. Shapiro, Ph.D.
Managing Director
The EROS Group, LLC