Bad performance / slow response when many concurrent sorts are executed
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Key: CORE-3989
URL: http://tracker.firebirdsql.org/browse/CORE-3989
Project: Firebird Core
Issue Type: Bug
Components: Engine
Affects Versions: 2.5.2, 2.1.5, 2.0.7, 2.5.1, 2.1.4, 2.5.0, 2.0.6, 3.0
Initial, 2.1.3, 2.1.2, 2.0.5, 2.1.1, 2.1.0
Environment: Reported and debugged on Linux, but generally
platform-independent
Reporter: Dmitry Yemanov
The issue manifests itself as slow server response under high load when many
concurrent connections are performing external sorts (i.e. PLAN SORT).
Backtrace shows that many threads are spending unexpectedly long time in system
calls mmap/munmap. Besides being a partucular connection's problem, it's also
blocking the AST delivery thus affecting other connections as well. In Classic
/ SuperClassic it may result to temporary (up to multiple seconds) server
freezes.
The problem is that the every sort (regardless of its size) needs 128KB of
memory for the first level buffer and such big allocations are redirected to
the operating system. With a high number of relatively small (read: cheap)
sorts the engine tends to spend more time mapping/unmapping the memory than
sorting the records.
The proposed solution is to cache a few recently used sort buffers and reuse
them in subsequent sorts rather than dealing with the system memory manager.
The field tesing has proved that being an effective measure.
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