Full_Name: Juerg Bircher Version: lmdb (master) OS: MacOS / Linux URL: ftp://ftp.openldap.org/incoming/Juerg_Bircher_160527-Improved-handling-for-large-number-of-databases.patch Submission from: (NULL) (178.82.37.195)
There is a increased performance penalty the more databases are created within the same environment. I was looking for a way the improve that by keeping the simplicity of tracking databases within a list with direct access by index (MDB_dbi). mdb_dbi_open() is however not improved with the assumption that the database handle (dbi) is cached in the application. So mdb_dbi_open() should happen only once for each database during the life time of an application.
One issue is that mdb_txn_begin() (for read-only transactions) calloc the sizeof(MDB_txn) + me_maxdbs * sizeof(MDB_db + 1). The plus 1 for the dbflags. However it is sufficient only to malloc that size and clear the sizeof(MDB_txn)
memset(txn, 0, sizeof(MDB_txn)
After that the data beyond the MDB_txn is not initialized which is ok for the moment.
The next improvement happens in mdb_txn_renew0() where the dbflags are only set to DB_UNUSED (a new flag) for each database currently opened in the environment.
memset(txn->mt_dbflags, DB_UNUSED, txn->mt_numdbs);
The former code used to to loop through each database to calculate the dbflags. This is still done but lazily for each accessed database with the assumption that a read only transaction rarely uses all databases of the environment.
The lazy initialization of the dbflag happens in the macro TXN_DBI_EXIST which is always used when a database handle (dbi) is passed to an function. The flags are updated in mdb_setup_db_info() once a database is access which is marked as unused (DB_UNUSED).
static int mdb_setup_db_info(MDB_txn *txn, MDB_dbi dbi) { /* Setup db info */ uint16_t x = txn->mt_env->me_dbflags[dbi]; txn->mt_dbs[dbi].md_flags = x & PERSISTENT_FLAGS; txn->mt_dbflags[dbi] = (x & MDB_VALID) ? DB_VALID|DB_USRVALID|DB_STALE : 0; return (txn->mt_dbflags[dbi] & validity); }
/** Check \b txn and \b dbi arguments to a function and initialize db info if needed */ #define TXN_DBI_EXIST(txn, dbi, validity) \ ((txn) && (dbi#C3C(txn)->mt_numdbs && (((txn)->mt_dbflags[dbi] & (validity)) || (((txn)->mt_dbflags[dbi] & DB_UNUSED) && mdb_setup_db_info((txn), (dbi), (validity)))))
The next improvement is done in any function which needs to loop through the databases for example in mdb_cursors_close(). Again the more databases in the environment the longer the execution time. It should be best if looping only through dbflags and searching for those databases which are used (!DB_UNUSED). This could be done byte wise or more efficient in 8/4 byte steps comparing with an extended mask DB_UNUSED_LONG instead of DB_UNUSED. So we can skip 8 or 4 (32 bit) unused databases in one step (still with the assumption that a transaction rarely uses all databases of the environment).
So the loop looks as follows always starting at the lower index to avoid alignment issues with ARM prior v6.
#define DB_UNUSED 0x20 /**< DB not used in this txn */
#ifdef MDB_VL32 #define DB_UNUSED_LONG 0x20202020 /* DB_UNUSED long mask for fast tracking */ #else #define DB_UNUSED_LONG 0x2020202020202020 /* DB_UNUSED long mask for fast tracking */ #endif
#ifdef MDB_VL32 #define MDB_WORD unsigned int #else #define MDB_WORD unsigned long long #endif
MDB_dbi %3= src->mt_numdbs; MDB_dbi i = 0;
while (1) { unsigned int upper = i + sizeof(MDB_WORD); if (upper < n) { // skip unused if ((*(MDB_WORD *)(tdbflags + i)) == DB_UNUSED_LONG) { i = upper; continue; } } else { upper = n; }
for (; i < upper; i++) { // any other filter criteria appropriate to the function .... } if (i >= n) { break; } }