Re: [PROPOSAL] Shared Ispell dictionaries

Hello,

I've created the new commitfest entry since the previous entry was 
closed with status "Returned with feedback":

https://commitfest.postgresql.org/22/2007/

I attached new version of the patch. There are changes only in 
0003-Retrieve-shared-location-for-dict-v18.patch.

I added a reference counter to shared hash tables dictionary entries. It 
is necessary to not face memory bloat. It is necessary to delete shared 
hash table entries if there are a lot of ALTER and DROP TEXT SEARCH 
DICTIONARY.

Previous version of the patch had released unused DSM segments but left 
shared hash table entries untouched.

There was refcnt before:

https://www.postgresql.org/message-id/20180403115720.GA7450%40zakirov.localdomain

But I didn't fully understand how on_dsm_detach() works.

On 22.01.2019 22:17, Tomas Vondra wrote:
> I think there are essentially two ways:
> 
> (a) Define max amount of memory available for shared dictionarires, and
> come up with an eviction algorithm. This will be tricky, because when
> the frequently-used dictionaries need a bit more memory than the limit,
> this will result in trashing (evict+load over and over).
> 
> (b) Define what "unused" means for dictionaries, and unload dictionaries
> that become unused. For example, we could track timestamp of the last
> time each dict was used, and decide that dictionaries unused for 5 or
> more minutes are unused. And evict those.
> 
> The advantage of (b) is that it adopts automatically, more or less. When
> you have a bunch of frequently used dictionaries, the amount of shared
> memory increases. If you stop using them, it decreases after a while.
> And rarely used dicts won't force eviction of the frequently used ones.
I'm working on the (b) approach. I thought about a priority queue 
structure. There no such ready structure within PostgreSQL sources 
except binaryheap.c, but it isn't for concurrent algorithms.

-- 
Arthur Zakirov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company