persistent_term to replace ETS for caching

[Frank Muller]

More info on the use-case:

The entries in the cache are created and deleted by a single unique
 process in the system. They can stay inside the cache for hours, days or
even weeks before being erased.

All other processes only read from cache.
There is no update to entries. Once they are created they stay unchanged
until they get deleted.

Does anyone know if the VM can efficiently create and handle millions of
persistent_term(s)?

/Frank

Sun 6  Dec 2020 17:40, Bob Ippolito <bob@REDACTED>  wrote :

> 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 <raoknz@REDACTED> 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 <ranjanified@REDACTED> 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 <frank.muller.erl@REDACTED>
>>> 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
>>>>
>>>
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