Re: [PATCH 1/3] Separate IRQ-stacks from 4K-stacks option

[Timothy Miller <miller@techsource.com>]

Lee Revell wrote:
> On Sun, 2004-09-12 at 18:07, Andrea Arcangeli wrote:
> 
>>On Sun, Sep 12, 2004 at 05:36:41PM -0400, Lee Revell wrote:
>>
>>>But in this case the hardirq handler can run for 2ms, which caused a
>>>scheduler latency problem, because nothing could run but other IRQs. 
>>>The IRQ threading in Ingo's patches solves the problem, and seems to me
>>>to be the correct solution.
>>
>>the irq threading must have a cost, doesn't it? I doubt you want to
>>offload irqs to a kernel thread on a server, *that* would be slow (irq
>>nesting is zerocost compared to scheduling a kernel thread).
> 
> 
> Yes, on a server you would probably disable threading for the disk and
> network IRQs (the VP patch lets you set this via /proc).  This feature
> effectively gives you IPLs on Linux, albeit only two of them.  For
> example to prevent heavy traffic on one network interface from impacting
> the latency of the other, you could enable threading for the first and
> disable it for the second.
> 
> I am still unsure why the IDE i/o completion is the one place that
> breaks the assumption that hardirq handlers execute quickly.


On the one hand, all this preemption introduces overhead which increases 
latency and reduces throughput for any one individual activity.

On the other hand, the preemption allows the CPU to attend to more 
different resources at the same time, keeping more resources busy. 
Under heavy load, this can increase overall system throughput significantly.

So, both approaches can increase throughput in different ways.  Where is 
the balance between the two which yields the most satisfactory result?