Oracle's Cursor Sharing for BMC Remedy Products - White Paper - www.bmc.com
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White Paper
Oracle's Cursor Sharing for
BMC® Remedy® Products
January 2007
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34 Oracle's Cursor Sharing for BMC Remedy Products
White Paper
Oracle's Cursor Sharing for BMC
Remedy Products
Cursor sharing has substantial benefits, and using it is a clear best practice for
BMC Remedy products. This white paper summarizes the expected benefits and
the risks from using this setting. Those already familiar with this technology can
go to “BMC Remedy product interaction and settings” on page 7 for specific usage
recommendations.
Oracle shared pool usage and functionality of
cursor sharing
Oracle's primary central memory structure is called the System Global Area (SGA).
The SGA has two primary parts:
! Buffer cache—Used to cache data
! Shared pool—Used to cache SQL statements
When an SQL statement is to be executed, a cursor is opened in the shared pool,
the statement is parsed, and it is associated with the open cursor and is ready to
execute. The next time that statement is encountered, if it is still in the shared pool
then it can be reused immediately (if the cursor is programmatically left open) and
after a soft parse operation (if the cursor has been closed). Scalable use of an Oracle
database depends on achieving good reuse of SQL statements, and soft parsing of
such SQL is very efficient. If the SQL statement cannot be reused, then it will again
be parsed entirely (called a hard parse), which is an expensive operation.
An SQL statement will have variables, and if these variables are hardcoded into the
text of the SQL statement, then they are called literal variables. If, on the other hand,
a variable is included in the SQL statement (in the format of :variable_name), and
that variable is instantiated before execution of the statement, then the statement
is using a bind variable. SQL statements that use literals are generally not reusable
as the shared pool stores a different copy of the SQL statement for each literal used
in the statement. These SQL statements cannot share an existing cursor because the
literal is changed during application execution. Conversely, statements with bind
variables are reused no matter how many different instantiations of the variable
there are, and hence result in better shared pool use.
Oracle's Cursor Sharing for BMC Remedy Products ! 5White Paper
Cursor_sharing is the Oracle technology that automatically replaces literal
variables with system-generated bind variables. Setting cursor_sharing to force
or similar enables sharing of literal SQL statements. When cursor_sharing is
enabled, the system generated bind variables are visible in the SQL being executed,
as literals are replaced with binds of the format ":sys_b_n", where n is an integer,
indicating a system-generated bind variable has been created.
The difference between force and similar is somewhat subtle, so it is generally
less important which setting to use. More important is to make sure that one of
these settings is enabled for an application that does not use bind variables. With
the setting of similar, SQL statements that are likely to change their execution plan
(especially when histograms are present) will not have automatically generated
bind variables used. Force will generate these bind variables even in such cases.
Performance effects of cursor sharing
Any application that uses all literals without cursor sharing enabled is not a very
scalable application. It is assumed that any good database application finds the
majority of its SQL statements available for reuse in a shared pool when they are
executed. Poor reuse of the shared pool means that Oracle must find a new space
to allocate a cursor within the shared pool every time a statement is encountered.
A least-recently-used (LRU) algorithm is used to determine what cursors should
be removed if Oracle needs to create new free space in the shared pool. As the
shared pool gets full with unshared statements, then the overhead in finding such
memory to free gets higher. Larger shared pools actually increase this overhead as
the larger shared pool becomes full and fragmented, and the LRU algorithm has
higher cost to evaluate. Applications that do not reuse SQL often run better with
smaller shared pools rather than larger ones.
Additionally, to remove existing cursors and open new ones, Oracle must latch
some memory structures. Specifically, it takes out library cache latches and shared
pool latches, which may result in latch contention in a database running such an
application.
Specific diagnostics that indicate poor reuse of SQL are found in statspack
(Oracle 8i and Oracle 9i) and AWR reports (oracle 10g). In the shared pool
summary on the first page of the report, the shared pool memory grows quickly
until its percentage used is roughly 90%. It never reaches 100%, as the LRU freeing
algorithm continues to remove cursors to make more space for new statements.
The next line in the report shows that the percentage of statements run more than
once (that is, >1) is relatively low, such as 20–40%. Similarly the amount of memory
allocated to statements that are run more than once is between 20% and 50%
(depending on how large the reused statements happen to be).
Next, such an application should show latch contention. In AWR reports, this is
stated explicitly as library cache and shared pool latch wait events in the wait event
list. The statspack indicates only that there is a latch free wait event, and then later
in the report, it indicates which latch is under contention (typically, library cache
latch first, and shared pool latch second).
6 "Oracle's Cursor Sharing for BMC Remedy ProductsOracle's Cursor Sharing for BMC Remedy Products
Enabling cursor_sharing significantly reduces the shared pool maintenance
overhead, as well as the latch contention overhead. The benefits for high-load
online transaction processing (OLTP) applications can be as much as 10% or 20%
CPU utilization reduction, as well as an inherently more scalable application
profile. The benefits will be higher for higher load applications, and may reduce
response time and provide better database performance overall.
BMC Remedy product interaction and settings
There is one significant conflict between the use of cursor_sharing and BMC
Remedy products. Inserts into a Long Raw column while cursor_sharing is
enabled can cause the BMC Remedy Action Request System® (AR System®) server
thread to hang. Such inserts are commonly done as part of the Email Engine
activity when attachments are included in email messages.
To avoid any impact from this issue, AR System server code explicitly disables
cursor_sharing (that is, sets it to exact) when doing such an insert into a Long Raw
column. However, it first needs to know that the current setting is not already
exact. To get the current setting, the AR System server looks in the ar.conf file,
where the cursor_sharing setting must be set to reflect the init.ora settings from
Oracle. If the ar.conf file does not reflect the init.ora settings, then the AR System
server assumes the value is already exact and, therefore, will not reset it when
doing inserts into Long Raw columns.
This issue was first fixed in AR System server version 6.0.1. However, the
workaround above was only done for the case of cursor_sharing=force. Users of
this server version should only set cursor_sharing=force and always reflect that
setting in ar.conf.
In versions 6.3 and 7.0, both settings cursor_sharing=force and
cursor_sharing=similar are respected, and the workaround is implemented for
both cases. So users of these product versions can use either setting, and once again
need to reflect that setting in the ar.conf file.
Best practices mandate specifically that, with 6.0.1, users should use force, and
with 6.3 or 7.0, users should use similar, but the difference between the two
settings is small.
The syntax of the cursor_sharing setting in the ar.conf file is as follows (set only
one):
! Oracle-Cursor-Sharing: SIMILAR
! Oracle-Cursor-Sharing: FORCE
Resolved defects related to cursor_sharing include:
! SW00168835
! SW00172819
! SW00202422
BMC Remedy product interaction and settings ! 7White Paper
! SW00209632
! SW00219469
8 "Oracle's Cursor Sharing for BMC Remedy Products*67831* *67831* *67831* *67831* *67831*
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