Re: [PATCH v2] tracing: Add linear buckets to histogram logic

[Steven Rostedt]

On Tue, 6 Jul 2021 16:20:07 -0700
Namhyung Kim <namhyung@xxxxxxxxxx> wrote:

> >  { bytes_req: ~ 1400-1499 } hitcount:         30
> >  { bytes_req: ~ 2000-2099 } hitcount:          6
> >  { bytes_req: ~ 4000-4099 } hitcount:       2168
> >  { bytes_req: ~ 5000-5099 } hitcount:          6  
> 
> For consistency with the log2 histogram, I'd like to see
> 
>   { bytes_req: ~ 100 } hitcount:  3149
>   { bytes_req: ~ 200 } hitcount:  1468
>   { bytes_req: ~ 300 } hitcount:    39
>   ...
> 
> Or, if you really care about the value range
> 
>   { bytes_req: 0 ~ 99 } hitcount:  3149
>   { bytes_req: 100 ~ 199 } hitcount:  1468
>   { bytes_req: 200 ~ 299 } hitcount:    39

(Let the bike-shedding begin! ;-)

I actually dislike the log2 notation. For example, I just ran it with
this:

 ># echo 'hist:keys=bytes_req.log2:sort=bytes_req' > events/kmem/kmalloc/trigger
 ># cat events/kmem/kmalloc/hist
 # event histogram
 #
 # trigger info: hist:keys=bytes_req.log2:vals=hitcount:sort=bytes_req.log2:size=2048 [active]
 #

 { bytes_req: ~ 2^5  } hitcount:          8
 { bytes_req: ~ 2^6  } hitcount:          2
 { bytes_req: ~ 2^7  } hitcount:          4
 { bytes_req: ~ 2^8  } hitcount:          2
 { bytes_req: ~ 2^9  } hitcount:          2
 { bytes_req: ~ 2^10 } hitcount:          3

 Totals:
     Hits: 21
     Entries: 6
     Dropped: 0

And I don't know if that first entry is: 2^4 - 2^5 or if it is 2^5 - 2^6.

And to me '~' means "approximately", but I also took it as "not exactly".
I used it as:

  { bytes_req: ~ 1400-1499 } hitcount:         30

To mean, it's "approximately somewhere between 1400 and 1499" so, I kept the "~".

Now for your suggestions:

>   { bytes_req: ~ 100 } hitcount:  3149
>   { bytes_req: ~ 200 } hitcount:  1468
>   { bytes_req: ~ 300 } hitcount:    39

Suffers the same fate as I dislike in log2. Is " ~ 100" 0-100 or 100-200?

>   { bytes_req: 0 ~ 99 } hitcount:  3149
>   { bytes_req: 100 ~ 199 } hitcount:  1468
>   { bytes_req: 200 ~ 299 } hitcount:    39

I feel is farther from log2 than my version. Stating that "~" means
approximation, what does "0 ~ 99" really mean?

So far I prefer my original version.

BTW, we are also working on a user space parser for this, thus the
output format of all hist logic is going to be a user space API (if it
hasn't already become one.)

So we do need to get this correct for the long haul.

-- Steve