Memory management plays a critical role in any operating system (OS), providing lower latency and higher throughput. Even though memory is volatile, it significantly reduces data access times compared to disk I/O. This is especially true for databases, where the size of memory can have a profound effect on performance. A crucial aspect of memory management is how the kernel manages memory pages. Here, we'll dive into how Oracle databases benefit from configuring Huge Pages, a feature that addresses the challenges of large memory environments.
The Role of Memory in an OS
Memory, while volatile, offers a faster alternative to disk storage for reading and writing data. The speed difference between memory I/O and disk I/O is significant, which is why databases benefit greatly from optimizing memory usage. The kernel handles memory in pages—small blocks of memory, typically 4KB in size, that are used to store data. For most applications, 4KB pages work well. However, databases often require more memory, and the management of these small pages can become a bottleneck.
How the Kernel Manages Memory Pages
At the core of memory management is the Memory Management Unit (MMU) within the CPU. The MMU stores the addresses of pages in a structure known as the page table. When dealing with small memory allocations, this system works well, but when memory scales up to terabytes, it introduces overhead. As an example, consider a server with 2TB of memory:
2 TB = 2 × 10^12 bytes / 4KB (page size) = 50 million pages
Managing 50 million pages creates overhead and can degrade system performance. For databases like Oracle that deal with large memory sizes, this is an inefficient system.
Addressing the 4KB Limit: Configuring Larger Pages
Oracle databases often need more than 4KB page sizes to function optimally. Yes, it is possible to configure larger pages—8KB, 16KB, or more. However, there's a trade-off: using larger page sizes for applications that don't require them wastes memory. Thus, the decision to configure pages larger than the default 4KB should be based on specific organizational requirements.
Huge Pages to the Rescue
It is a specialized memory structure created by the kernel to manage larger blocks of memory. Huge Pages are especially useful for applications, like Oracle databases, that work with large amounts of data. In systems like Oracle Exadata, where memory runs into terabytes and the default block size is 8KB, Huge Pages optimize memory management and reduce the overhead of handling large volumes of small pages.
What Happens if Huge Pages Aren't Used in Oracle?
Oracle databases by default use an 8KB block size. Without Huge Pages, the MMU must manage multiple page table entries for each database block, adding to the system’s memory management burden. This can degrade performance, particularly in Exadata systems where memory can scale to several terabytes. The Linux kernel’s Huge Pages feature offers a solution by providing larger page sizes that better match the block sizes used by Oracle databases.
The default size for Huge Pages is 2MB, a significant upgrade from the standard 4KB.
Configuring Huge Pages: Key Considerations
When configuring Huge Pages, there are some critical points to keep in mind. One of the most important is ensuring that the total System Global Area (SGA) size across all Oracle databases running on a server does not exceed the available number of Huge Pages. If it does, the database will fail to start due to a lack of memory. For example, if the collective SGA size is 100GB, you will need: 100GB / 2MB (Huge Page size) = 51,200 Huge Pages
Failing to account for this will result in performance issues or database startup failures.
How Oracle Databases Use Huge Pages in Exadata and Non-Exadata Systems
The database’s use of Huge Pages is governed by the use_large_pages parameter, which can be set to several values:
TRUE: The database will first allocate memory from Huge Pages, then fall back to standard memory pages.
FALSE: The database will not use Huge Pages. Oracle strongly advises against this for large SGAs as it negatively affects performance.
ONLY: The database will exclusively use Huge Pages, and if they are not available, the system will fail to start. This is the most recommended setting by Oracle.
AUTO: The database will automatically configure the necessary Huge Pages and use standard pages if needed. This eliminates DBA-to-sysadmin coordination but is not recommended.
AUTO_ONLY: Like AUTO, but the database will only use the available Huge Pages.
Steps to Configure Huge Pages
Huge Pages usually require a server reboot unless configured by the root user. Below are the basic steps to enable Huge Pages:
[root@dbahelp ~]# echo "vm.nr_hugepages=2048" >> /etc/sysctl.conf
[root@dbahelp ~]# cat /etc/sysctl.conf
vm.nr_hugepages=2048
[root@dbahelp ~]# sysctl -p
vm.nr_hugepages = 2048
[root@dbahelp ~]# cat /proc/meminfo | grep -i huge
anonhugepages: 178176 kb
hugepages_total: 850
hugepages_free: 850
hugepages_rsvd:
hugepages_surp:
hugepagesize: 2048 kb
hugetlb: 1730560 kbIn this example, 850 Huge Pages are configured, providing 1890 MB of memory for the SGA.
Conclusion
For Oracle DBAs, Huge Pages offer a vital performance boost, especially in systems with large memory allocations like Exadata. Proper configuration ensures that databases can fully utilize available memory without overwhelming the MMU or wasting resources. By understanding the nuances of memory management and configuring Huge Pages effectively, you can significantly enhance your Oracle database performance.
This blog was designed to provide a deep dive into how Huge Pages work, how to configure them, and why they matter in the context of Oracle databases. Effective memory management is the cornerstone of database performance, and Huge Pages are an invaluable tool in a DBA’s arsenal.
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