On 30. mai 2016 13:51, Hallvard Breien Furuseth wrote:

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Maybe a good way in between could be to memcpy txn->mt_env->me_dbflags. (...) memcpy uses SSE instructions for x86_64.

Yes. If we make MDB_txn.mt_dbflags an uint16_t*, we can memcpy to it. That costs <maxdbs> bytes/txn + one more reserved env flag bit in PERSISTENT_FLAGS: Reserve DB_UNUSED, and keep it always set in me_dbflags so the memcpy will set it in mt_dbflags.

Oops, that was too clever. Then a bunch of code which assumes a DBI is valid if various txn flags are set, must also check DB_UNUSED. Or invert that to DB_USED which might be easier - (flags & DB_DIRTY) becomes F_ISSET(flags, DB_DIRTY|DB_USED) which checks if both are set.

Maybe the simple solution using 2*<maxdbs> bytes/txn would be better.