Re: gettimeofday() vsyscall for kvm-clock?

[Andrew Theurer <habanero@linux.vnet.ibm.com>]

On 05/21/2012 03:36 PM, Marcelo Tosatti wrote:
> On Mon, May 21, 2012 at 03:26:54PM -0500, Andrew Theurer wrote:
>> Wondering if a user-space gettimofday() for kvm-clock has been
>> considered before.  I am seeing a pretty large difference in
>> performance between tsc and kvm-clock.  I have to assume at least
>> some of this is due to the mode switch for kvm-clock.  Here are the
>> results:
>>
>> (this is a 16 vCPU VM on a 16 thread 2S Nehalem-EP host, looped
>> gettimeofday() calls on all vCPUs)
>>
>> tsc:		.0645 usec per call
>> kvm-clock: 	.4222 usec per call (6.54x)
>>
>>
>> -Andrew Theurer
>
> https://bugzilla.redhat.com/show_bug.cgi?id=679207
>
> "model name : Intel(R) Xeon(R) CPU           E5540  @ 2.53GHz
>
> native, gettimeofday (vsyscall): 45ns
> guest, kvmclock (syscall): 198ns"
>
> But this was before
>
> commit 489fb490dbf8dab0249ad82b56688ae3842a79e8
> Author: Glauber Costa<glommer@redhat.com>
> Date:   Tue May 11 12:17:40 2010 -0400
>
>      x86, paravirt: Add a global synchronization point for pvclock
>
> (see the full changelog for details).
>
> Can you try disabling the global variable, to see if that makes
> a difference (should not be enabled in production)? Untested patch
> (against guest kernel) below

The following was re-done on a 3.4 guest kernel (previously RHEL kernel):

1-way:
   tsc:		.0315
   kvm-clock:	.2112 (6.7x)

16-way:
   tsc:		.0432
   kvm-clock: 	.4825 (11.1x)

Now with global var disabled:

16-way:
   kvm-clock:	.4628

Does not look like much of a difference.

-Andrew