On 4/24/14 3:58 AM, Bart Vandewoestyne wrote:
Hello list,I noticed that for most (if not all) of my queries, the first time the query is run (right after i started the virtuoso server), it takes much longer than all subsequent runs. See e.g. my query timings at https://dl.dropboxusercontent.com/u/32340538/long_first_query.png Using dstat monitoring, I noticed that during these long first query runs, there is a lot of reading from disk. I have some questions concerning this: * What is the phenomenon I'm seeing here? What is happening? Why is this first run so much slower than all subsequent ones?
Basic DBMS behavior in regards to caching i.e., the first execution is typically more expensive (from your perspective) because the query solution data is making its way into the working set cache.
For instance, with the LOD cloud cache we host [1], following DBMS server startup, we run a series of queries to get the engine warmed up (i.e., getting records from disk to working set cache).
Once the engine is warmed up, cache invalidation (yet another piece of fundamental DBMS functionality) takes care of working set management until the DBMS server is shutdown.
* Can this 'warmup' phenomenon be reduced somehow so that also the first queries run as fast as the subsequent ones?
No.Think about this as being similar to 0 - 60 mph when dealing with the speed of a car. In a DBMS, the more memory you have the more data you pack into the working set cache, subject to I/O between disk and memory.
* Is this phenomenon documented somewhere? Is there a place where I can read and learn more about it?
This is basic DBMS functionality. Kingsley
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