Running H2 on a Raspberry PI I noticed a CPU usage of 5% even when nothing
was going on in the database. I found that a "MVStore background writer"
was active every 50 milliseconds. Debugging the cause I came across this
line
int sleep = Math.max(1, millis / 10);
in MVStore's setAutoCommitDelay method, which at least could nearly explain
the high frequency wake-ups given a default commit delay of 1000 I thought
to have read about, except for a factor of two.
I noticed that MVStore.setAutoCommitDelay is called by my first query
through JdbcConnectionPool.getConnection(), and from MVStore.<init> where
the connection string is
jdbc:h2:file:/home/harald/data/ekl/ekldb;TRACE_LEVEL_SYSTEM_OUT=1
with a hard coded value of millis=1000. Shortly later it is called again
through Database.setWriteDelay with millis taken from Database.writeDelay
hardcoded to Constants.DEFAULT_WRITE_DELAY, which is 500.
This finally explained the 50 milliseconds wakeups. But two questions arise:
1) Why is the background writer set twice?
2) Why the division by 10 of the millis? How about targeting the delta by
90% with a first delay and then delay for the missing delta? (I assume you
tried an old fashioned java.util.Timer and it was not up to the task?)
I'll try setting the WRITE_DELAY to a higher value to not drain unnecessary
CPU power on the PI, since risking a data loss on this PI database is no
big deal.
Harald.
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