The shared cache does not know that the table is small nor that there is nothing else to load.
When a thread accesses that shared cache, it must protect itself from the data page it's on being modified, either because the page is simply flushed from the cache (if the cache does such things) or replaced by another page because this one happened to be the least recently used (or whatever rule the cache uses). (As with others, I haven't actually looked at the code.) -----Original Message----- From: sqlite-users-boun...@sqlite.org [mailto:sqlite-users-boun...@sqlite.org] On Behalf Of Boris Kolpackov Sent: Monday, August 13, 2012 8:51 AM To: sqlite-users@sqlite.org Subject: Re: [sqlite] Multi-Thread Reads to SQLite Database Hi Richard, Richard Hipp <d...@sqlite.org> writes: > In shared-cache mode, the page cache is shared across threads. That > means that each thread must acquire a mutex on the page cache in order > to read it. Which means that access to the page cache is serialized. I just ran our concurrency test in different configurations and I observer a similar behavior. That is, in the shared-cache mode, read-only transactions on the same table are executed pretty much sequentially. Also, your explanation doesn't feel quite satisfactory to me. In his original email, Eric mentioned that his table contains just 50 rows. Surely all this data would be loaded into the cache the first time it is requested and then accessed concurrently by all the threads. The only way I can see how the sequential performance could be explained here is if the cache mutex did not distinguish between readers and writers (which would seem to be a fairly natural thing to do). In our test, on the other hand, each thread queries its own set of rows from the table. So, based on your explanation, here each thread should end up with its own set of pages (more or less). However, even in this case, I still observe a near sequential performance. Any idea what else might be going on here? Boris -- Boris Kolpackov, Code Synthesis http://codesynthesis.com/~boris/blog Compiler-based ORM system for C++ http://codesynthesis.com/products/odb Open-source XML data binding for C++ http://codesynthesis.com/products/xsd XML data binding for embedded systems http://codesynthesis.com/products/xsde _______________________________________________ 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
