Re: Perfmon systemtap runtime support

[William Cohen <wcohen at redhat dot com>]

Chuck Ebbert wrote:
> In-Reply-To: <44B6C74E.7040008@redhat.com>
>
> On Thu, 13 Jul 2006 18:21:02 -0400, William Cohen wrote:
>
> > I have been working on getting some performance monitoring support into 
> > systemtap. The perfmon1.diff patch is a very simple addition to the 
> > runtime. It just has functions to setup the perfmon monitoring hardware, 
> > read a counter, and shutdown the performance monitoring hardware. It 
> > uses the perfmon2 kernel ABI to configure the hardware.
> >
> > I have completed changes to the translator to use the runtime functions. 
> > I took Marin's suggestion of using guru mode to allow access to the 
> > various C functions and wrote some examples that used the runtime functions.
> >
> > The cost is relatively high for accessing the counters. Below is the 
> > output from p2x.stp, counting the number of cycles between consecutive 
> > calls to read the cycle count:
> >
> > [wcohen@dhcp59-187 systemtap_perfmon]$ ./install/bin/stap -g  p2x.stp
> > interval = 15491
>
>
> I modified your sample pure C module from a while ago and I get 500-700
> cycles overhead:

I tried tried your example and I got similar numbers:

Jul 14 11:47:40 dhcp59-187 kernel: val0 = 6595400, val1 = 6595847, 
interval = 447
Jul 14 11:47:41 dhcp59-187 kernel: val0 = 6842567, val1 = 6843068, 
interval = 501

I don't know why I got such high cycle counts yesterday. Running the 
same p2x.stp today I got lower cycle counts:

[wcohen@dhcp59-187 systemtap_perfmon]$ ./install/bin/stap  -g p2x.stp
interval = 1051
[wcohen@dhcp59-187 systemtap_perfmon]$ ./install/bin/stap  -g p2x.stp
interval = 1009
[wcohen@dhcp59-187 systemtap_perfmon]$ ./install/bin/stap  -g p2x.stp
interval = 650
[wcohen@dhcp59-187 systemtap_perfmon]$ ./install/bin/stap  -g p2x.stp
interval = 1063
[wcohen@dhcp59-187 systemtap_perfmon]$ ./install/bin/stap  -g p2x.stp
interval = 983

-Will