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xen-devel
Re: [Xen-devel] irqbalance?
At 18:13 17/07/2007, pak333@xxxxxxxxxxx wrote:
resending as i did not receive a response. Anyone out there familiar
with irq code
thanks
Prabha
Hi All,
If I turn off irqbalance xen/arch/x86/irq.c, does that actually stop
XEN from balancing irqs across different physical cpus, or will that
override the setting if there are too many interrupts and one of the
cpus is overloaded.
I don't know this, but it makes little sense to have a feature to
turn something off if it's not actually turning it off. How would you
like it if your lightswitches in the house decided to turn on the
lights because it was too dark?
Example: i have 4 cpus and i have configured irqbalance= off, so
there is no irqbalancing done by xen.
That is what I'd expect.
Now if i have affintized all my physical interrupts to one cpu say
cpu1 all the interrutps should be handled by the cpu1 and so should
all the softirqs generated.
Now what happens if one of the other cpus is lightly loaded, will
some of the softirqs be queued against the other cpus or will cpu1
handle all interrupts and softirqs.
I'd like to point out that (at least in my understanding) softirq's
and "real" irq's aren't strictly related, so you could well have a
SOFTIRQ as a result of a hard IRQ on a different CPU - that's again a
"my understanding", rather than something I've read in the code, but
I believe my understanding is correct.
Also, to the best of my understanding, setting the IRQ affinity would
essentially be the same as turning irq-balance off (for that interrupt).
Looks to me like there are two levels of interrupt redirection, one
done by the hardware where each physical device interrupt is
assigned to a particual processor dynamically and then on top of
that is the irqbalancing done by Xen itself. Is that true? and
irqbalance=off only affects the latter and interrupt affinitzation
is for the former?
Affinity is a setting to restrict an interrupt (or other types of
processing, e.g. threads, processes or VMs) to a certain processor -
irq-balancing is "the opposite", e.g. that it attempts to move IRQ's
around so that they are not all happening on a particular processor -
this can be done in several different ways, either spreading one
particular interrupt between the available processors, or by
spreading several interrupts, so for example a Network interrupt
happens always on one CPU, but another CPU takes disk interrupts, a
third CPU takes all the timer interrupts, etc.
There's no firm and fast rule of which is better (obviously, if
interrupts happen VERY frequently, then the cache would work better
if they are all on one CPU). Also, if you have a NUMA architecture
where the time to "get to" a certain device is dependant on which
physical CPU the code is running on, then it would be better for the
IRQ to be on the CPU "nearest" to the device.
All of these options have advantages and drawbacks.
I guess this is still related to your scheduler questions previously,
but I'm not certain I understand what you're actually trying to
achieve: preventing interrupts from being processed may give you
better performance for the CPU-bound tasks, but in general it's
better for OVERALL performance of the system if the I/O performance
is improved - as it generally has very small impact on the CPU
intensive performance, but I/O performance can easily be very badly
affected by delays (latency) added to the path of the IO-request.
--
Mats
Thanks
Prabha
From: pak333@xxxxxxxxxxx
To: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-devel] irqbalance?
Date: Mon, 16 Jul 2007 23:17:44 +0000
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