Re: Some questions

[Akira Yokosawa <akiyks@gmail.com>]

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On 2016/04/29 8:05, Akira Yokosawa wrote:
> On 2016/04/29 1:28, Paul E. McKenney wrote:
>> On Fri, Apr 29, 2016 at 12:39:09AM +0900, Akira Yokosawa wrote:
>>> On 2016/04/27 16:28:07 -0700, Paul E. McKenney wrote:
>>>> On Thu, Apr 28, 2016 at 08:01:31AM +0900, Akira Yokosawa wrote:
>>>>> On 2016/04/27 15:50:22 -0700, Paul E. McKenney wrote:
>>>>>> On Thu, Apr 28, 2016 at 07:15:05AM +0900, Akira Yokosawa wrote:
>>>>>>> On 2016/04/27 09:53:57 -0700, Paul E. McKenney wrote:
>>> [snip.
>>>>>>>> Please see attached for what it looks like to me.
>>>>>>>
>>>>>>> Well, this is identical to the one I built.
>>>>>>> So, do you intend to explicitly put numbers which show up fairly long time, and
>>>>>>> leave other cells blank even below changes of values denoted by (n) in italics?
>>>>>>
>>>>>> The blank cells represent cache misses.  The CPU is waiting for a read
>>>>>> to complete during that time.  A non-blank cell corresponds to a CPU
>>>>>> actually completing a read.
>>>>>
>>>>> Oh, I see. But this should be explained in the text, I think.
>>>>
>>>> Good point!  I also added several other possibilities, including
>>>> interrupts and preemption.
>>>>
>>>
>>> So, I have a few questions regarding to the added explanation of blank cells.
>>>
>>> According to the text, trace data used to create the table are said to be 
>>> obtained by a program that contains the code fragment shown in Figure 14.4.
>>> The loop in the code fragment will exit once it sees state.variable != mycpu.
>>> That means the actual program you used has an outer loop to record the
>>> changes of state.valuable for each cpu in the system, I suppose.
>>> Am I guessing right?
>>
>> IIRC, the program just stuffed timestamps and values into a set of per-CPU
>> big arrays, then printed them out.  A script took these values as input,
>> and compacted identical state.
>>
>>> If I am, (n)'s in the table denoting modification of variables must be
>>> entries in the trace data which were output from the outer loop, I think.
>>
>> The (n)'s mark changes in value for a given CPU.
>>
>>> However, in the table, there are a number of cases where (n)'s are followed
>>> by blanks just below itself. Does this mean fetched state.variables stay in
>>> the cache very briefly, but are (almost immediately) invalidated by a cache
>>> coherence mechanism? I can see interrupts and preemption would also cause the
>>> trace output to be suspended for a while.
>>
>> It marks places where a given CPU saw a value momentarily.  As you say,
>> this could be due to cache invalidation, interrupts, preemption, etc.
>>
>>> I'm not sure I have made out what the table means thus far, but am I seeing
>>> something close enough to what you intend the table to represent?
>>
>> The main point is that different CPUs can disagree on the value of a given
>> variable at a given point in time.  The following diagram shows that
>> this disagreement is nevertheless bounded, in that all CPUs must agree
>> on the ordering of values for that variable.
> 
> Yes, of course that's the main point.
> I should have asked in a different way.
> 
> There should be the same kind of situations in Figure 14.5.
> But you didn't depict them in the figure.
> 
> Why did you put the blank cells in the table in the first place?
> 
> I'm a little bit distracted by those blank cells, and began questioning
> about them.
> 
> Isn't it enough to just do the same way in Table 14.2 as in Figure 14.5?
> 
> Or, could the blank cell situation be explained in the form of an answer
> of a quick quiz?  It would be much easier for me to grasp the main point
> of this section "Variables Can Have More Than One Value" while reading the
> body of the text.
> 
> Thoughts?
> 
>                                                    Thanks, Akira
> 
>>
>> 							Thanx, Paul
>>
>>
> 

So I drew 2 figures based on Table 14.2.
No, I didn't actually drew them but wrote a rough program to generate .fig
format files from the value changes of each CPU extracted from Table 14.2.
I ignored the blank cells in the table just as in Figure 4.5.
Appended is a tar ball of two files.
out.fig is the overall diagram, and out-2.fig is a zoomed in view of
the beginning part.
I think they can provide a fairly interesting view of what's going on.

Please give a look at them.

                                                    Thanks, Akira

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