Re: [Xen-devel] [PATCH] tools/libxc: Avoid hypercalls for cacheflush on x86

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-03-04 at 11:22 +0000, Andrew Cooper wrote:
> On 04/03/14 01:14, Ian Campbell wrote:
> > On Mon, 2014-03-03 at 16:22 +0000, Ian Jackson wrote:
> >> Andrew Cooper writes ("[PATCH] tools/libxc: Avoid hypercalls for 
> >> cacheflush on x86"):
> >>> XEN_DOMCTL_cacheflush hypercalls are -ENOSYS on x86.  As domain
> >>> creation/migration is already very hypercall-heavy, avoid making extra
> >>> redundant hypercalls
> >> Surely this should be done by moving the relevant code to an
> >> arm-specific file, not by #ifdeffery.
> > I think it is very debatable that sort-circuiting a single ENOSYS
> > hypercall in the tools is even worth it at all. "very hypercall heavy"
> > doesn't cut it as a rationale IMHO. Unless an actual demonstrable
> > improvement is seen, perhaps over a larger series removing a large
> > number of so called redundant calls, it's just an additional source of
> > confusion (due to gating at multiple levels) as far as I'm concerned.
> >
> > Ian.
> >
> 
> For booting a single guest, noone will notice.  Booting 10 or more at a
> time however is very different.
> 
> XenServer, along with all other virtualisation platforms live in a world
> where "Net time to boot $N guests" (where N is measured in hundreds) is
> an important quantity, and needless overhead from things like this cause
> dom0 needless load that it would rather do without.

That's fine.

> I should say that I have not done any performance metrics on this - as
> noted in the patch I stumbled upon it with valgrind.

And in the absence of such performance metrics I see no reason to apply
this patch, which was what my initial reply said.

Ian.


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