Re: [Xen-devel] [RFC PATCH] xen: free_domheap_pages: delay page scrub to tasklet

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.05.14 at 09:11, <bob.liu@xxxxxxxxxx> wrote:
> On 05/20/2014 02:27 PM, Jan Beulich wrote:
>> So if you have the system scrub 1Tb at boot (via suitable
>> dom0_mem=), how long does that take?
>> 
> 
> I only have a 32G machine, the 1Tb bug was reported by our testing engineer.
> 
> On 32G machine, if set dom0_mem=2G the scrub time in "(XEN) Scrubbing
> Free RAM:" is around 12s at boot.
> 
> The xl destroy time for a 30G guest is always around 15s even decreased
> the rate of calling hypercall_preempt_check().

Okay, so these numbers at least appear to correlate. And in fact I
think 3Gb/s (approximated) isn't that unreasonable a number; at
least it's not orders of magnitude away from theoretical bandwidth.

Which means yes, better dealing with the load resulting from the
post-guest-death scrubbing would be desirable, but otoh it's also
not really unexpected for this taking minutes for huge guests. Any
change here clearly need proper judgment between latency and
the effect on other guests it has: As said previously, impacting all
other guests just so that the scrubbing would get done quickly
doesn't seem right either.

Jan


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