[DTrace-devel] min() and max() aggregation map initialization

[Eugene Loh]

Incidentally, my answer from last night was not quite right.

I was suggesting iterating over the aggregations with dt_idhash_iter().  
Then, there would be some callback function with a dt_ident_t pointer, 
call it idp.  I think idp->di_kind should be DT_IDENT_AGG for each one.

But what kind of aggregation function does it use?  I suggested looking 
at idp->di_id, but the problem with that is it's the ID that identifies 
the aggregation, not the aggregation function it uses.  E.g., if you 
have "@a = count()", then idp->di_id identifies "@a", while you want to 
know about "count()".

Apparently, you can still get at the aggregation function.  I mean, you 
have to be able to since "@a = count(); @a = max(1)" is illegal.  So, we 
have to be able to check that the aggregation function associated with 
an aggregation is always consistent.  The code for that is in 
dt_parser.c;  look for the string "aggregation redefined".  It gives 
away the dirty secret:  di_iarg!  Oh, so counterintuitive.  Anyhow, you can:

         dt_ident_t *aggfunc = idp->di_iarg;

Then, you can check that the aggfunc is min or max by comparing 
aggfunc->di_id to DT_AGG_MIN or DT_AGG_MAX (or comparing 
aggfunc->di_name to "min" or "max", but that's probably a bad idea!).

On 12/02/2020 09:01 PM, Eugene Loh wrote:
> Totally guessing here, but maybe it'll get you started.  Maybe where you
> describe, you can  dt_idhash_iter(dtp->dt_aggs,...). Then check the
> di_kind for each ident and look for DT_IDENT_AGG. If so, check di_id for
> what kind of aggregation it is.  If it's MIN or MAX, you can get the
> offset from di_offset, and then initialize both values (di_offset and
> di_offset + 8).  I've never done any of this, so I have no idea if it's
> right.
>
>
> On 12/02/2020 07:46 PM, david.mclean at oracle.com wrote:
>> I noticed during my testing that min() and max() need to be initialized
>> to non-zero values before they are used.
>> For example, it is a problem if the initialized value is zero for max()
>> and after that only negative values are fed to max() -- the result will
>> be a value of zero falsely populating the final mapped values with an
>> incorrectly large value.
>>
>> Eugene pointed me to using dt_bpf_map_update() to populate these
>> initialization values, but I don't know where in the code base I would
>> want to issue the function to initialize the values.  I've been spending
>> my time in the aggs and _impl functions and I don't know my way around
>> enough to easily find the preferred place for the call.
>>
>> My guess is I should add something toward the end of
>> dt_bpf_gmap_create(), maybe right after the other dt_bpf_map_update()
>> call, and find a way to detect which maps belong to min() or max()
>> aggregations.
>>
>> I figure for min() I would want to initialize to the highest value
>> (0x7FFFFFFFFFFFFFFF) and for max() I should initialize the lowest value
>> (0x8000000000000000).
>>
>> Some guidance to speed this up would be appreciated.
>>
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