Hi,
Saw this today on an xrootd mailing list.
Has anyone seen anything about this before? Tried it?
Yours,
Chris.
On 04/04/2011 15:52, "Charles G Waldman" <[log in to unmask]> wrote:
>
> > Are any of xrootd team aware of
> > http://twiki.mwt2.org/bin/view/ITB/UltraLightKernel ? are there any
>(up
> > to date) recommendations based on this guys work?
>
>Hi -
>
> I'm involved with both xrootd and the configuration of the Ultralight
>kernel.
>
> The major difference between the UltraLight kernel and the stock SL
>kernel is that the SL kernel is configured with preemption enabled,
>which is great for a user desktop, but not for a high-throughput
>data server.
>
> According to the kernel configuration docs:
>
> CONFIG_PREEMPT_NONE:
>
> This is the traditional Linux preemption model, geared towards
> throughput. It will still provide good latencies most of the
> time, but there are no guarantees and occasional longer delays
> are possible.
>
> Select this option if you are building a kernel for a server or
> scientific/computation system, or if you want to maximize the raw
> processing power of the kernel, irrespective of scheduling
> latencies.
>
>This is what Ultralight uses.
>
>
>The SL/Redhat kernel is configured with
>
> CONFIG_PREEMPT:
>
> This option reduces the latency of the kernel by making all kernel
> code (that is not executing in a critical section) preemptible.
> This allows reaction to interactive events by permitting a low
> priority process to be preempted involuntarily even if it is in
> kernel mode executing a system call and would otherwise not be
> about to reach a natural preemption point. This allows
> applications to run more 'smoothly' even when the system is under
> load, at the cost of slightly lower throughput and a slight runtime
> overhead to kernel code.
>
> Select this if you are building a kernel for a desktop or embedded
> system with latency requirements in the milliseconds range.
>
>We have found that on our test server (16-core server Dell 2950 with 6
>RAID6 shelves) with the SL kernel, under load, the "ksoftirqd" daemon
>takes up 100% CPU (this is a kernel thread involved in preemption),
>and the xrootd process is only able to deliver data at about 500MB/sec
>(about 50% of theoretical max on a on a 10Gb link).
>
>With the UL kernel, the "ksoftirq" CPU usage goes away completely, and
>xrootd is easily able to deliver data at wire speed (~1000MB/sc)
>
> - Charles
>
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