On 02/28/2018 11:45 PM, Simon Slavin wrote:
On 1 Mar 2018, at 7:24am, Shevek <[email protected]> wrote:
What I think is happening is that either a pthread mutex or a database lock is
serializing the accesses, so each thread blocks the others.
To be specific, I'm concerned about is the line
sqlite3_mutex_enter(db->mutex) at the top of sqlite3_step(). Since my
queries are spending all their time in sqliteVdbeExec(), which is
reached through that path, I assume db->mutex is preventing concurrency.
Our main hotspots in the query are sqliteVdbeExec() and updating the
btree pointer to point to a new page (I forget the call name). We can't
do much about the cost of execution; we've mmap'd everything to avoid
the I/O, we're using covering indexes to help with locality, we've
sorted our query keys to attempt to reduce index page seeks, and now we
want to use concurrency and splitting the logic in our query to exploit
memory bandwidth.
Now I've traced this again, I'm looking warily at SQLITE_OPEN_NOMUTEX
because we need thread-safety, as in, sqlite's internal data structures
must be handled correctly in the presence of multiple threads or passing
a connection between threads (safely in the JMM); we just don't need
serialization of database reads and writes, because nothing we do has a
serializable side-effect. Is SQLITE_OPEN_NOMUTEX the answer?
What journal mode are you using ?
I'm fairly sure journal mode is NONE for our readonly database. Anyway,
readonly shouldn't write to a journal. We have confirmed that the md5sum
of the database file is unchanged during and after the execution of our
application.
S.
<https://sqlite.org/pragma.html#pragma_journal_mode>
If it's not WAL, try WAL. If it's currently WAL, try DELETE. Once you've
changed it see if this changes how your program behaves.
Simon.
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