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Diffstat (limited to 'doc/build/faq/performance.rst')
| -rw-r--r-- | doc/build/faq/performance.rst | 50 |
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diff --git a/doc/build/faq/performance.rst b/doc/build/faq/performance.rst index c6f51ebb1..d6ee557ea 100644 --- a/doc/build/faq/performance.rst +++ b/doc/build/faq/performance.rst @@ -460,53 +460,3 @@ Script:: test_sqlite3(100000) -.. _faq_compiled_cache_threshold: - -How do I deal with "compiled statement cache reaching its size threshhold"? ---------------------------------------------------------------------------- - -Some parts of the ORM make use of a least-recently-used (LRU) cache in order -to cache generated SQL statements for fast reuse. More generally, these -areas are making use of the "compiled cache" feature of :class:`.Connection` -which can be invoked using :meth:`.Connection.execution_options`. - -The following two points summarize what should be done if this warning -is occurring: - -* Ensure the application **does not create an arbitrary number of - Engine objects**, that is, it does not call :func:`.create_engine` on - a per-operation basis. An application should have only **one Engine per - database URL**. - -* If the application does not have an unbounded number of engines, - **report the warning to the SQLAlchemy developers**. Guidelines on - mailing list support is at: http://www.sqlalchemy.org/support.html#mailinglist - -The cache works by creating a cache key that can uniquely identify the -combination of a specific dialect and a specific Core SQL expression. -A cache key that already exists in the cache will reuse the already-compiled -SQL expression. A cache key that doesn't exist will create a *new* entry -in the dictionary. When this dictionary reaches the configured threshhold, -the LRU cache will trim the size of the cache back down by a certain percentage. - -It is important to understand that from the above, **a compiled cache that -is reaching its size limit will perform badly.** This is because not only -are the SQL statements being freshly compiled into strings rather than using -the cached version, but the LRU cache is also spending lots of time trimming -its size back down. - -The primary reason the compiled caches can grow is due to the **antipattern of -using a new Engine for every operation**. Because the compiled cache -must key on the :class:`.Dialect` associated with an :class:`.Engine`, -calling :func:`.create_engine` many times in an application will establish -new cache entries for every engine. Because the cache is self-trimming, -the application won't grow in size unbounded, however the application should -be repaired to not rely on an unbounded number of :class:`.Engine` -objects. - -Outside of this pattern, the default size limits set for these caches within -the ORM should not generally require adjustment, and the LRU boundaries -should never be reached. If this warning is occurring and the application -is not generating hundreds of engines, please report the issue to the -SQLAlchemy developers on the mailing list; see the guidelines -at http://www.sqlalchemy.org/support.html#mailinglist. |