In Oracle, memory management revolves around two main areas:
- SGA (System Global Area): This shared memory region contains data and control information for the Oracle instance. It includes the Buffer Cache (similar to SQL Server’s Buffer Pool), shared SQL areas, and other structures.
- PGA (Program Global Area): This is a private memory region allocated for each individual Oracle session or process. It includes session-specific data and control information.
A lower PGA cache hit ratio indicates that Oracle is frequently unable to find required data blocks in the PGA cache, necessitating more frequent disk I/O operations to retrieve data from disk into memory. Consequently, queries that heavily rely on PGA memory for sorting, joins, and other operations may execute more slowly.
Additionally, increased CPU utilization may occur as Oracle spends more processing time managing memory operations instead of executing queries.
Symptoms
Users typically experience slower query response times and degraded overall database performance.
Impact: High
In multi-user environments, contention for resources such as memory and CPU can intensify, potentially leading to performance bottlenecks and locking issues.
Expected behavior
While there isn’t a specific universally agreed-upon target for the PGA cache hit ratio that guarantees optimal performance (as it can vary based on workload, system configuration, and application requirements), a PGA cache hit ratio above 90% is typically considered good. This means that 90% or more of the time, Oracle can satisfy data requests from the PGA memory, minimizing the need for costly disk I/O operations.
Recommendations
Monitoring and optimizing PGA memory usage, adjusting relevant Oracle parameters, and tuning SQL queries are crucial steps to mitigate these issues and improve overall database performance and user satisfaction.
AimBetter alerts when the PGA cache hit ratio is below the set threshold and provides tools to identify and improve the performance of non-optimal queries.
