RFC (round 1), JEP draft: Low-level Object layout introspection methods

[Peter Levart]

Hi,

On 8/17/20 5:13 PM, Erik Österlund wrote:
> Perhaps, another way of answering the same question without the 
> addressOf API, is to have an estimatedDistanceInBytes(Object o1, 
> Object o2) or even an
> estimatedDistanceInBytes(Object o1, Field f1, Object o2, Field f2) 
> API. This API could run in a mode where there are no
> safepoints, and ensure that none of the above mentioned "impossible" 
> situations actually remain impossible, and hence more effectively 
> actually answering the high-level question.
> It would also importantly never expose any addresses or offsets, while 
> still allowing various locality heuristics to be computed by 
> performance people. 

...still, if you wanted to obtain the estimated distances among all 
pairs of (object, field)-s in a set of objects, individually query-ing 
for each pair would not give you a snapshot view for the whole set. If 
this is important, one would perhaps have to have an API like this:

record ObjectField(Object object, Field field) {}

long[][] estimatedDistancesInBytes(ObjectField[] objectFields)

this API returns a matrix of distances (let's say lower triangle without 
diagonal), a total of n*(n-1)/2 distances for n ObjectField instances. 
This requires quadratic space. For 1M objects, the API would return ~ 
1/2 a trillion distances (4T bytes). This might be a problem.

one could argue that if distances are a signed number (not absolute), so 
that distance(f1, f2) == -distance(f2, f1), the API could also be like:

long[] estimatedDistancesInBytes(ObjectField[] objectFields)

where the i-th element d[i] of the returned array would represent a 
distance(f[i], f[i+1]). To get a distance from f[x] to f[y] (y >= x), 
you would then just sum(d[i]; x <= i <= y).

OTOH, an API like:

long[] estimatedAddressesOf(ObjectField[] objectFields)

would also work and would not require summing y-x+1 numbers to get a 
distance from f[x] to f[y] but just calculate one difference: a[y] - 
a[x]. If addresses are mangled (each is added the same random offset), 
the API doesn't expose any more of internals than alternative APIs that 
returns distances. I would also say that exposing distance(s) admits 
existence of points with location (i.e. addresses) so even 
philosophically you don't expose more internals.


Regards, Peter