Note 1557506 - Linux paging improvements
Symptom
You experience high swapping activity on your Linux system, e.g. when triggering a filesystem backup
or similiar, although the SAP applications are sized to completely fit into the system's main memory.
This results in high response times on the application level as well.
Other terms
SUSE, SLES, SLES for SAP, Enterprise Server, suse, SuSE, Novell, novell, LINUX, linux, Page
Cache, page cache, pagecache, filesystem cache, pagecache swapping, swapping, swappiness,
paging, swap partition, swap file, pagecache_limit_mb, page cache limit, memory usage, poor
performance, performance problem, bad performance
Reason and Prerequisites
This problem is known to exist on all Linux operating systems.
SAP solutions can need large amounts of memory to process business data in memory. Besides the
memory needed for the applications, the Linux Kernel is using the remaining free memory for other
purposes such as the page cache. The Linux page cache keeps data read from hard disk in main
memory to speed up access in case the same data is accessed again. It also caches writes to the
filesystem resulting in better write throughput and in hiding latencies for disk writes.
The Linux page cache (this is called filesystem cache on some other systems) is the reason why a
Linux system will always show only a small amount of "free" memory after running for a certain time,
since every access to hard disk is kept in the page cache as long as there is enough free memory
available. The Linux kernel will shrink the Page Cache automatically, if applications need more
memory.
(For more details, please search resources on the Internet, like http://linux-mm.org/Low_On_Memory,
as well.)
If the remaining memory available for application data ("free" plus page cache) gets low, the Linux
kernel can decide to page out currently inactive memory into the swap partition / swapfile in order to
free-up main memory. This typically improves the performance of the system, since unused memory
areas don't use the limited main memory anymore. The freed-up memory can be used e.g. by
applications -- or by the page cache in order to improve I/O performance. The Linux Kernel decides
which memory areas are paged out, based on a LRU (least recently used) algorithm. Recently used
memory areas stay in the main memory. Not recently used memory areas may be paged out to the
disk.
Faced with filesystem load such as backup jobs, the Linux kernel may decide to prefer the page cache
over SAP application memory areas, if these memory areas have not recently been used (depending
on the memory activities this does not necessarily mean hours). It pages out these memory areas to
the disk (into the swap partition or swapfile). When the application then tries to access a memory
region that has been pagedout, the response time is poor, since these memory areas have to be read
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