Re: persistent_term to replace ETS for caching

I’m not familiar with the implementation details but my understanding from
the docs is that erase/1 and put/2 (only when updating an existing term)
cause a global GC. What processes do with those terms is only relevant in
that if they have references to multi-word persistent terms that are being
updated or erased then the global GC pause will take longer.

On Sun, Dec 6, 2020 at 06:29 Richard O'Keefe <[email protected]> wrote:

> Thanks for that advice about persistent_term.
> From the documentation,
> <quote>
> When a persistent term is updated or deleted, a global garbage collection
> pass is run to scan all processes for the deleted term, and to copy it into
> each process that still uses it.
> </quote>
> Do I understand correctly that if three processes refer
> to a persistent-term, and one of them deletes it, it
> will be copied into both of the other processes, thus
> INCREASING the amount of memory used?
> This is sufficiently counter-intuitive that I am not
> sure I would dare to use this feature.
>
> Do I further understand correctly that *adding* a new
> persistent-term does NOT force garbage collection?
>
>
> On Mon, 7 Dec 2020 at 00:49, Nalin Ranjan <[email protected]> wrote:
>
>> It has the potential to trigger Global GC, and can affect responsiveness
>> as per the docs.
>>
>> https://erlang.org/doc/man/persistent_term.html
>>
>> Regards
>> Nalin Ranjan
>>
>> On Sun, Dec 6, 2020 at 5:16 AM Frank Muller <[email protected]>
>> wrote:
>>
>>> Hi guys,
>>>
>>> At work, we cache about 5.3 million entries in ETS. The system works
>>> perfectly, no issue so far (many years).
>>>
>>> During a brainstorming session, a colleague suggested to switch to
>>> persistent_term instead to avoid ETS term copying.
>>>
>>> Pretty simple: we check if the Key exists in persistent_term. If yes, we
>>> are done. If not, we get it from ETS, move it to persistent_term and send
>>> it back to the caller.
>>>
>>> Question: is there any limitation(s) on persistent_term usage? Stated
>>> otherwise, can we create 5.3 million persistent_term <K,V>?
>>>
>>> Any suggestion/idea/thought is very welcome.
>>>
>>> /Frank
>>>
>>