RE: [Xen-devel] [PATCH] TSC scaling for live migration between platforms with different TSC frequecies

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> Meh.  "RDTSC exiting" is Intel's term for it in the SDMs.

Well Intel's choice of term in some obscure corner of
the SDM doesn't seem like a good choice when it is
misleading to mortals.  "Exiting" means "leaving" or
"quitting", so mortals probably would read this as
that RDTSC ("whatever that is" sez the mere mortal)
is shutting down.  Not to mention the fact that
"enabling XXX exiting" sounds like an oxymoron. :-)

> Not if it's going to print that out every time I
> migrate a VM.  This whole feature is something I'd
> turn off anyway.

You'd turn it off at your peril unless you are
certain all the potential migration target machines
in your data center have identical TSC rates
and/or all of your guests don't have any reliance
on TSC for keeping time.

The message should only print if you ARE migrating
between machines with different TSC rates, not for
every migration.  Since the consequence of turning
it on is a potentially large loss in performance,
a clear log message seems a small price to pay.

Dan

> -----Original Message-----
> From: Tim Deegan [mailto:Tim.Deegan@xxxxxxxxxx]
> Sent: Thursday, June 18, 2009 10:04 AM
> To: Dan Magenheimer
> Cc: Patrick Colp; Keir@xxxxxxxxxxxxxxxxxxxx;
> xen-devel@xxxxxxxxxxxxxxxxxxx; Zhang, Xiantao; Keir Fraser
> Subject: Re: [Xen-devel] [PATCH] TSC scaling for live 
> migration between
> platforms with different TSC frequecies
> 
> 
> At 16:45 +0100 on 18 Jun (1245343507), Dan Magenheimer wrote:
> > >       gdprintk(XENLOG_INFO, "Loaded VM expects a 
> %"PRIu32"MHz TSC "
> > >                "but CPU is %ldMHz; enabling RDTSC exiting.\n",
> > >                hdr->gtsc_khz / 1000, cpu_khz / 1000); 
> > 
> > RDTSC "exiting"?  Do you mean RDTSC emulation?
> 
> Meh.  "RDTSC exiting" is Intel's term for it in the SDMs.
> 
> > Also, frankly, given the potential performance 
> ramifications, perhaps
> > this should be higher than XENLOG_INFO?
> 
> Not if it's going to print that out every time I migrate a VM.  This
> whole feature is something I'd turn off anyway.
> 
> Cheers,
> 
> Tim.
> 
> > > -----Original Message-----
> > > From: Zhang, Xiantao [mailto:xiantao.zhang@xxxxxxxxx]
> > > Sent: Thursday, June 18, 2009 3:47 AM
> > > To: Tim Deegan; Patrick Colp
> > > Cc: Keir@xxxxxxxxxxxxxxxxxxxx; 
> xen-devel@xxxxxxxxxxxxxxxxxxx; Fraser
> > > Subject: RE: [Xen-devel] [PATCH] TSC scaling for live 
> > > migration between
> > > platforms with different TSC frequecies
> > > 
> > > 
> > > Tim Deegan wrote:
> > > > At 10:10 +0100 on 18 Jun (1245319857), Patrick Colp wrote:
> > > >>> +        printk("Migrate to a platform with different 
> > > freq:%ldMhz, "
> > > >>> +            "expected freq:%dMhz, enable rdtsc exiting!\n",
> > > >>> +                    cpu_khz / 1000, hdr->gtsc_khz / 1000);
> > > >> 
> > > >> Being pedantic, this should probably be:
> > > >> 
> > > >>      printk("Migrate to a platform with different 
> freq: %ldMHz, "
> > > >>             "expected freq: %dMHz, enable rdtsc exiting!\n",
> > > >>              cpu_khz / 1000, hdr->gtsc_khz / 1000);
> > > > 
> > > > Being _pedantic_, it should be
> > > > 
> > > >       gdprintk(XENLOG_INFO, "Loaded VM expects a 
> %"PRIu32"MHz TSC "
> > > >                "but CPU is %ldMHz; enabling RDTSC exiting.\n",
> > > >                hdr->gtsc_khz / 1000, cpu_khz / 1000); 
> > > 
> > > It maybe wonderful to add VM info (eg. Domain id) as Jan says 
> > > in another mail. Thanks for your suggestions! I will change 
> > > it in the final version!  :-)
> > > Xiantao
> > > 
> > > 
> > > _______________________________________________
> > > Xen-devel mailing list
> > > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > > http://lists.xensource.com/xen-devel
> > >
> 
> -- 
> Tim Deegan <Tim.Deegan@xxxxxxxxxx>
> Principal Software Engineer, Citrix Systems (R&D) Ltd.
> [Company #02300071, SL9 0DZ, UK.]
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-devel
>

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