Re: [patch 0/3] [Announcement] Performance Counters for Linux

[Ingo Molnar <mingo@elte.hu>]

* David Miller <davem@davemloft.net> wrote:

> From: Ingo Molnar <mingo@elte.hu>
> Date: Fri, 5 Dec 2008 08:03:29 +0100
> 
> > 
> > * Ingo Molnar <mingo@elte.hu> wrote:
> > 
> > > This can be done in a very natural way with our abstraction, and the 
> > > "hello.c" example happens to do exactly that:
> > 
> > multiple people pointed out that we have not posted hello.c :-/
> 
> Because it's completely not providing the facility.  This is not how
> people want to use the performance counters at all.
> 
> And it doesn't even do what Paulus said is necessary, he said:
> 
> --------------------
> > One thing that this sort of thing can't do is to get values from 
> > multiple counters that correlate with each other.  For instance, we 
> > would often want to count, say, L2 cache misses and instructions 
> > completed at the same time, and be able to read both counters at very 
> > close to the same time, so that we can measure average L2 cache misses 
> > per instruction completed, which is useful.
> --------------------
> 
> And if you read one counter then read the other as seperate operations, 
> you get extra events in there as a side effect of going back into 
> userspace between the two reads.

that's wrong. If you _want_ to measure in a different context, with as 
little measurement impact as possible, you can do it with our code. The 
announcement provides the example for that.

For example, i just started this bash infinite loop:

  $ while :; do :; done &
  [1] 1877

  $ ./monitor -e 1 -c 1000000000 1877
  IP: 0x00000031a2e70d4b
  IP: 0x0000000000455f64
  IP: 0x00000031a2f028a0
  IP: 0x0000000000440692
  IP: 0x0000000000441b8e
  IP: 0x00000031a2e6f630
  IP: 0x0000000000446129
  IP: 0x00000031a2e6edbc
  IP: 0x0000000000443736
  IP: 0x0000000000441c80
  IP: 0x000000000043913a
  ^C

We get IP readouts every 1 billion instructions executed in that shell. 
That shell is never stopped or otherwise intruded - it's kept as an as 
pristine of an execution environment as possible.

Furthermore, the event readouts strictly only include event counts of the 
shell PID, _not_ of the monitor context's read() or other activities.

> Nobody wants that, [...]

Nobody wants that and we dont do it.

Really, you should take a more serious look at our code.

	Ingo