Starvation in RAFContainer.readPage()
-------------------------------------
Key: DERBY-733
URL: http://issues.apache.org/jira/browse/DERBY-733
Project: Derby
Type: Bug
Components: Performance, Store
Versions: 10.2.0.0, 10.1.2.1, 10.1.3.0, 10.1.2.2
Environment: Solaris x86 and Linux with Sun JVM 1.5.0. Derby embedded and
client/server.
Reporter: Knut Anders Hatlen
Assigned to: Knut Anders Hatlen
When Derby is completely disk bound, threads might be starved in
RAFContainer.readPage(). This is a real problem when multiple clients
are repeatedly accessing one or a small number of large tables. In
cases like this, I have observed very high maximum response times
(several minutes in the worst cases) on simple transactions. The
average response time is not affected by this.
The starvation is caused by a synchronized block in
RAFContainer.readPage():
synchronized (this) {
fileData.seek(pageOffset);
fileData.readFully(pageData, 0, pageSize);
}
If many threads want to read pages from the same file, there will be a
long queue of threads waiting for this monitor. Since the Java
specification does not guarantee that threads waiting for monitors are
treated fairly, some threads might have to wait for a long time before
they get the monitor. (Usually, a couple of threads get full throughput
while the others have to wait.)
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