Re: mdb_dbi_open and threads

[Hallvard Breien Furuseth <h.b.furuseth@usit.uio.no>]

On 21. mai 2017 21:43, Muhammed Muneer wrote:
> Howard Chu wrote
>
> "Just follow the recommendation to open all handles at the beginning of the 
> program."
>
> But what if I have lots of named databases like maybe 10000 or more. Wouldn't 
> this be expensive.

Yes.  From ldmb.h:

"Currently a moderate number of slots are cheap but a huge number gets
expensive: 7-120 words per transaction, and every #mdb_dbi_open()
does a linear search of the opened slots."

> So in a multi threaded environment, if the name of a named dbi is generated 
> from within a write
> transaction (thread1) and proceeds to mdb_dbi_open it only to find that 
> another read transaction
> (thread 2) just opened the same named dbi after the write-txn of thread 1 
> started, the prospect of
> mdb_dbi_open the same named dbi for thread 1 is lost forever.

With threads 1 and 2 coexisting? When thread 2 called mdb_dbi_open(),
thread 1's prospect of using mdb_dbi_open() at all was lost.

--
Hallvard