Re: [perfmon2] comments on Performance Counters for Linux (PCL)

[Ingo Molnar]

* Corey Ashford <cjashfor@xxxxxxxxxxxxxxxxxx> wrote:

> Just a few comments below on some excerpts from this very good discussion.
>
> Peter Zijlstra wrote:
>> On Thu, 2009-05-28 at 16:58 +0200, stephane eranian wrote:
>>>      - uint64_t irq_period
>>>
>>>        IRQ is an x86 related name. Why not use smpl_period instead?

irq is not an x86 related name at all. There's thousands of uses of 
it even in arch/powerpc:

  earth4:~/tip> git grep -i irq arch/powerpc/ | wc -l
  6441

>>
>> don't really care, but IRQ seems used throughout linux, we could 
>> name the thing interrupt or sample period.
>
> I agree with Stephane, the name irq_period struck me as somewhat 
> strange for what it does.  sample_period would be much better.

sample_period would be fine - but smpl_period definitely not ;-)

>>>      - uint32_t record_type
>>>
>>>        This field is a bitmask. I believe 32-bit is too small to 
>>>        accommodate future record formats.
>>
>> It currently controls 8 aspects of the overflow entry, do you 
>> really forsee the need for more than 32?
>
> record_type is probably not the best name for this either.  Maybe 
> "record_layout" or "sample_layout" or "sample_format" (to go along 
> with read_format)

'record' is pretty established for this - so record_layout would be 
fine. Peter?

>>>        I would assume that on the read() side, counts are accumulated as
>>>        64-bit integers. But if it is the case, then it seems there is an
>>>        asymmetry between period and counts.
>>>
>>>        Given that your API is high level, I don't think tools should have to
>>>        worry about the actual width of a counter. This is especially true
>>>        because they don't know which counters the event is going to go into
>>>        and if I recall correctly, on some PMU models, different counters can
>>>        have different width (Power, I think).
>>>
>>>        It is rather convenient for tools to always manipulate counters as
>>>        64-bit integers. You should provide a consistent view between counts
>>>        and periods.
>>
>> So you're suggesting to artificually strech periods by say 
>> composing a single overflow from smaller ones, ignoring the 
>> intermediate overflow events?
>>
>> That sounds doable, again, patch welcome.
>
> I definitely agree with Stephane's point on this one.  I had 
> assumed that long irq_periods (longer than the width of the 
> counter) would be synthesized as you suggest.  If this is not the 
> case, PCL should be changed so that it does, -or- at a minimum, 
> the user should get an error back stating that the period is too 
> long for the hardware counter.

this looks somewhat academic - at least on x86, even the fastest 
events (say cycles) with a 32 bit overflow means one event per 
second on 4GB. That's not a significant event count in practice. 
What's the minimum width we are talking about on Power?

	Ingo
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