Re: [Xen-devel] [RFC PATCH] Start PV guest faster

[Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>]

On Fri, 2014-05-30 at 07:54 +0100, Jan Beulich wrote:
> >>> Frediano Ziglio <frediano.ziglio@xxxxxxxxxx> 05/29/14 7:24 PM >>>
> >On Tue, 2014-05-20 at 10:30 +0100, Jan Beulich wrote:
> >> >>> On 20.05.14 at 09:26, <frediano.ziglio@xxxxxxxxxx> wrote:
> >> > It's a while I noticed that the time to start a large PV guest depends
> >> > on the amount of memory. For VMs with 64 or more GB of ram the time can
> >> > become quite significant (like 20 seconds). Digging around I found that
> >> > a lot of time is spend populating RAM (from a single hypercall made by
> >> > xenguest).
> >> 
> >> Did you check whether - like noticed elsewhere - this is due to
> >> excessive hypercall preemption/restart? I.e. whether making
> >> the preemption checks less fine grained helps?
> >> 
> >
> >Yes, you are right!
> >
> >Sorry for late reply, I got some time only now. Doing some tests with a
> >not so bug machine (3gb) and using strace to see amount of time spent
> >
> >| Xen preempt check | User allocation | Time all ioctls (sec) |
> >| yes               | single pages    | 0.262                 |
> >| no                | single pages    | 0.0612                |
> >| yes               | bunk of pages   | 0.0325                |
> >| no                | bunk of pages   | 0.0280                |
> >
> >So yes, preemption check (I disable entirely for the tests!) is the main
> >factor. Of course disabling entirely is not the right solution. Are
> >there some way to understand how often to do, some sort of
> >computation/timing?
> 
> If you look at other instances, it's mostly heuristic at this point. I suppose
> you'd want to make the preemption granularity slightly allocation order
> dependent, e.g. preempt every 64 pages allocated (unless, of course, the
> allocation order is even higher than that).
> 
> Generally a time based approach (say every millisecond) might be
> reasonable too, but reading out the time on each iteration isn't without
> cost, so I'd recommend against this.
> 
> Jan
> 

I was thinking at some adaptive approach where at beginning you try to
understand how much to wait. In the meantime I got results from my
tests.

See
https://docs.google.com/spreadsheets/d/1u_93KXr1SGfn4Pz47tjPA71MMPnnQ4rTPF3SOANd_oY/edit?usp=sharing

For the test I disabled entirely preemption.

Frediano



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