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.
>
>
>
>
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