Re: [Xen-devel] Tracking down a boot speed issue

[Roger Cruz <roger.cruz@xxxxxxxxxx>]

A question remains in my mind as why the times are variable.  Why does it take 
.67 secs and 1.6 secs other times for exactly the same boot code?
________________________________________
From: Ian Campbell [Ian.Campbell@xxxxxxxxxx]
Sent: Friday, October 26, 2012 2:38 PM
To: Roger Cruz
Cc: xen-devel@xxxxxxxxxxxxx
Subject: Re: [Xen-devel] Tracking down a boot speed issue

On Fri, 2012-10-26 at 18:30 +0100, Roger Cruz wrote:
> As you see from the timed samples below, the access can take anywhere
> from .6 to 1.2 secs (though I have seen it take 1.6 secs every single
> time with  Linux 3.2.16 and Xen 4.0.3).

Isn't this just because setting up the per-cpu shared info makes the
time available to printk. Note that the timestamp is zero beforehand
(a big hint that time isn't setup yet). Also see xen_clocksource_read().

IOW I think what is being measured here is the time from start of day
until the second printk, not the time between the two prints.

[...]
>             printk(KERN_ALERT "in cpu loop %d for shared info", cpu);
> ===>    per_cpu(xen_vcpu,cpu) = &HYPERVISOR_shared_info->vcpu_info[cpu];
>             printk(KERN_ALERT "in cpu loop %d computed shared info", cpu);
>
> Oct 25 20:24:11 23445LU-PBBFMWY kernel: [    0.000000] in cpu loop 0 for 
> shared info
> Oct 25 20:24:11 23445LU-PBBFMWY kernel: [    1.238877] in cpu loop 0 computed 
> shared info
>
> Oct 25 20:52:53 23445LU-PBBFMWY kernel: [    0.000000] in cpu loop 0 for 
> shared info
> Oct 25 20:52:53 23445LU-PBBFMWY kernel: [    0.718365] in cpu loop 0 computed 
> shared info
>
> Oct 25 20:41:27 23445LU-PBBFMWY kernel: [    0.000000] in cpu loop 0 for 
> shared info
> Oct 25 20:41:27 23445LU-PBBFMWY kernel: [    0.671690] in cpu loop 0 computed 
> shared info
>
>
>
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