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Re: [Xen-devel] softirq bound to vcpus

I think I got the subject of the email wrong, it looks like it is by real cpu
not vcpu? 


On Wed, 15 Mar 2006 13:42:09 -0600
Tim Freeman <tfreeman@xxxxxxxxxxx> wrote:

> In "Understanding the Linux Kernel" 3rd edition, section 4.7 "Softirqs and
> Tasklets" it states: 
> "Activation and execution [of defferable functions] are bound together: a
> deferrable function that has been activated by a given CPU must be executed on
> the same CPU. There is no self-evident reason suggesting that this rule is
> beneficial for system performance. Binding the deferrable function to the
> activating CPU could in theory make better use of the CPU hardware cache.
> After all, it is conceivable that the activating kernel thread accesses some
> data structures that will also be used by the deferrable function. However,
> the relevant lines could easily be no longer in the cache when the deferrable
> function is run because its execution can be delayed a long time. Moreover,
> binding a function to a CPU is always a potentially "dangerous" operation,
> because one CPU might end up very busy while the others are mostly idle."
> I think in Xen, the same approach is taken from what I can tell
> (softirq_pending is keyed by cpu, used both in activation (cpu_raise_softirq)
> and execution (do_softirq)).
> Is there room for optimization here?  I'm looking at Xen's softirq mechanism
> and just noticed this, that's all.  I have no performance data or anything. 
> In Xen, when would the hardware cache situation in the quote above generally
> have a real effect?  Are there any particularly large data structures accessed
> with defferable functions in Xen?
> Tim 
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