Does SQLite support MVCC (http://en.wikipedia.org/wiki/Multiversion_concurrency_control)? It sounds like it doesn't. Maybe it should--that's a very nice way to support unblocked reads while still getting mutex for writes.
----- Original Message ----- From: "Dan" <danielk1...@gmail.com> To: "General Discussion of SQLite Database" <sqlite-users@sqlite.org> Sent: Monday, July 27, 2009 9:27 AM Subject: Re: [sqlite] SQLite lock behavior in shared-cache mode > > While compiling any statement (sqlite3_prepare_v2()), or while > stepping > (sqlite3_step()) a statement that accesses the main database, a > mutex > associated with the in-memory cache of the main database will be > held. > > Dan. > > > > On Jul 27, 2009, at 11:16 PM, Alessandro Merolli wrote: > >> Hi, >> >> I'm using SQLite latest version (3.6.16) with shared-cache enable >> in >> a process that has around 5 threads. Database connections for each >> thread are created with the same main database file. After that, >> each >> connection is attached to a particular database file (one for each >> thread) using the same schema name. Final structure is similar to >> the >> following: >> >> Main database file: main.db >> Thread-1 database file: thread1.db >> Thread-2 database file: thread2.db >> Thread-3 database file: thread3.db >> Thread-4 database file: thread4.db >> Thread-5 database file: thread5.db >> >> Thread-1 connection is opened with the main.db file and attaches >> the >> thread1.db as "extradb" schema name; >> Thread-2 connection is opened with the main.db file and attaches >> the >> thread2.db as "extradb" schema name; >> Thread-3 connection is opened with the main.db file and attaches >> the >> thread3.db as "extradb" schema name; >> Thread-4 connection is opened with the main.db file and attaches >> the >> thread4.db as "extradb" schema name; >> Thread-5 connection is opened with the main.db file and attaches >> the >> thread5.db as "extradb" schema name; >> >> Every SQL statement submitted to the process and passed to one of >> these threads can read global informations maintained in the >> main.db >> database file ("main" schema) and write/read particular >> informations >> in the "extradb" schema in such a way that one thread does not need >> to >> wait for another thread to write its information, since each thread >> has the "extradb" schema attached to a particular database file. >> >> Shared-cache is used for 2 reasons: >> - to improve main.db database file data access; and, >> - use the "Unlock Notification Feature" (sqlite3_unlock_notify) to >> avoid many SQLITE_LOCKED errors based on the code provided in >> http://www.sqlite.org/unlock_notify.html >> . >> >> In my understanding, the expected behavior should be: >> - If a SQL statement with only read (heavy) operations is passed to >> Thread-1; and, >> - Another SQL statement with a write (and some reads) operations is >> passed to Thread-3; >> - Both should run in parallel in a multi-core system. >> >> But, it seems that the Thread-3 is waiting for the Thread-1 to >> finish >> its work before continue. >> This behavior is turning the solution into a non-scalable solution. >> >> As far as I could debug (and understand) using Visual Studio 2005, >> it >> seems that Thread-3 (in the above example) is waiting in for a lock >> in >> the sqlite3BtreeEnterAll function. See the piece of the call stack >> below: >> >>> sqlite3.dll!winMutexEnter(sqlite3_mutex * p=0x012fb2d8) Line >>> 15159 >> sqlite3.dll!sqlite3BtreeEnterAll(sqlite3 * db=0x1bb05ac8) Line >> 36706 + 0x11 bytes >> sqlite3.dll!sqlite3LockAndPrepare(sqlite3 * db=0x00000000, const >> char * zSql=0x00bbbbc0, int nBytes=180, int saveSqlFlag=1, >> sqlite3_stmt * * ppStmt=0x00000000, const char * * >> pzTail=0x0965f63c) >> Line 9672 >> sqlite3.dll!sqlite3_prepare_v2(sqlite3 * db=0x1bb05ac8, const char >> * zSql=0x00bbbbc0, int nBytes=180, sqlite3_stmt * * >> ppStmt=0x013a9094, >> const char * * pzTail=0x0965f63c) Line 9747 + 0x1f bytes >> apr_dbd_sqlite3.dll!sqlite3_blocking_prepare_v2(sqlite3 * >> db=0x00000000, const char * zSql=, int nSql=, sqlite3_stmt * * >> ppStmt=0x00000000, const char * * pz=0x00000000) Line 247 + 0x10 >> bytes >> apr_dbd_sqlite3.dll!dbd_sqlite3_select(apr_pool_t * >> pool=0x013a9050, apr_dbd_t * sql=0x029cc040, apr_dbd_results_t * * >> results=0x0965f688, const char * query=0x00bbbbc0, int seek=0) >> Line >> 307 + 0x33 bytes >> libaprutil-1.dll!apr_dbd_select(const apr_dbd_driver_t * >> driver=0x00a66270, apr_pool_t * pool=0x013a9050, apr_dbd_t * >> handle=0x029cc040, apr_dbd_results_t * * res=0x0965f688, const char >> * >> statement=0x00bbbbc0, int random=0) Line 319 + 0x22 bytes >> >> While Thread-1 call stack looks like this: >> >>> sqlite3.dll!sqlite3VdbeExec(Vdbe * p=0x1c471318) Line 52862 >> sqlite3.dll!sqlite3Step(Vdbe * p=0x00000000) Line 49388 + 0x7 >> bytes >> sqlite3.dll!sqlite3_step(sqlite3_stmt * pStmt=) Line 49449 + 0x7 >> bytes >> apr_dbd_sqlite3.dll!sqlite3_blocking_step(sqlite3_stmt * >> pStmt=0x00000000) Line 220 + 0x6 bytes >> apr_dbd_sqlite3.dll!dbd_sqlite3_select(apr_pool_t * >> pool=0x01340958, apr_dbd_t * sql=0x012e1e28, apr_dbd_results_t * * >> results=0x09b5f688, const char * query=0x0139c1b8, int seek=0) >> Line >> 324 + 0x5 bytes >> libaprutil-1.dll!apr_dbd_select(const apr_dbd_driver_t * >> driver=0x00a66270, apr_pool_t * pool=0x01340958, apr_dbd_t * >> handle=0x012e1e28, apr_dbd_results_t * * res=0x09b5f688, const char >> * >> statement=0x0139c1b8, int random=0) Line 319 + 0x22 bytes >> >> Additional information about the SQLite library in use: >> - Version 3.6.16 (amalgamation source) >> - Preprocessor definitions used to build the library: >> - SQLITE_THREADSAFE=1 >> - TEMP_STORE=3 >> - SQLITE_DEFAULT_CACHE_SIZE=65568 >> - SQLITE_DEFAULT_TEMP_CACHE_SIZE=65568 >> - SQLITE_MAX_ATTACHED=30 >> - SQLITE_ENABLE_COLUMN_METADATA >> - SQLITE_ENABLE_UNLOCK_NOTIFY >> >> >> Any observation or suggestion about this behavior is appreciated. >> Thanks for your time. >> >> Regards, >> Alessandro Merolli. >> >> >> >> >> _______________________________________________ >> sqlite-users mailing list >> sqlite-users@sqlite.org >> http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users > > _______________________________________________ > sqlite-users mailing list > sqlite-users@sqlite.org > http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users