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SubjectRe: RT patch acceptance
Andi Kleen wrote:
> Are you sure it is not only disk IO? In theory updatedb shouldn't
> need much CPU, but it eats a lot of memory and causes stalls
> in the disk (or at least that was my interpration on the stalls I saw)
> If there is really a scheduling latency problem with updatedb
> then that definitely needs to be fixed in the stock kernel.

I don't know, Debian's updatedb always seemed to suck up most of the CPU
for me. I am using ReiserFS with tail-packing on, which certainly
balances on the side of more CPU vs IO. Also I wouldn't be surprised if
other distros had some better approach than Debian's, which appears to
be a series of "find | sort" commands. As one would expect, find causes
most of the system load and sort causes user load spikes.
That said, preempt-RT is certainly not free right now. Sending network
messages at 60Hz appears to load this 2GHz system by about 8%, while
that workload barely shows up in stock. I figure there's still some
optimization work to be done, but obviously it's unlikely to ever be as
efficient as non-preempt-RT. The more interesting question is whether
it's any slower with the RT patch applied, but preemption turned off.
From the implementation approach, I don't think it will show any
difference from stock, but it's certainly something we've got to test a
fair amount to be sure.
- Jim Bruce
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