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SubjectRe: [PATCH v2] tracing: Add linear buckets to histogram logic
On Tue, 6 Jul 2021 16:20:07 -0700
Namhyung Kim <namhyung@kernel.org> 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

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