Anonymous memory tends to be program heap and stack memory (for example, >malloc()). It is reclaimable, except in special cases such as mlock or if there is no available swap space. Anonymous memory must be written to swap before it can be reclaimed. Swap I/O (both swapping in and swapping out pages) tends to be less efficient than pagecache I/O, because of allocation and access patterns.

A cache of file data. When a file is read from disk or network, the contents are stored in pagecache. No disk or network access is required, if the contents are up-to-date in pagecache. tmpfs and shared memory segments count toward pagecache.

When a file is written to, the new data is stored in pagecache before being written back to a disk or the network (making it a write-back cache). When a page has new data not written back yet, it is called “dirty”. Pages not classified as dirty are “clean”. Clean pagecache pages can be reclaimed if there is a memory shortage by simply freeing them. Dirty pages must first be made clean before being reclaimed.

This is a type of pagecache for block devices (for example, /dev/sda). A file system typically uses the buffercache when accessing its on-disk metadata structures such as inode tables, allocation bitmaps, and so forth. Buffercache can be reclaimed similarly to pagecache.

Buffer heads are small auxiliary structures that tend to be allocated upon pagecache access. They can generally be reclaimed easily when the pagecache or buffercache pages are clean.

As applications write to files, the pagecache (and buffercache) becomes dirty. When pages have been dirty for a given amount of time, or when the amount of dirty memory reaches a specified number of pages in bytes (vm.dirty_background_bytes), the kernel begins writeback. Flusher threads perform writeback in the background and allow applications to continue running. If the I/O cannot keep up with applications dirtying pagecache, and dirty data reaches a critical setting (vm.dirty_bytes), then applications begin to be throttled to prevent dirty data exceeding this threshold.

The VM monitors file access patterns and may attempt to perform readahead. Readahead reads pages into the pagecache from the file system that have not been requested yet. It is done to allow fewer, larger I/O requests to be submitted (more efficient). And for I/O to be pipelined (I/O performed at the same time as the application is running).

Applications running on openSUSE Leap 42.2 can allocate more memory compared to openSUSE Leap 10. This is because of glibc changing its default behavior while allocating user space memory. See http://www.gnu.org/s/libc/manual/html_node/Malloc-Tunable-Parameters.html for explanation of these parameters.

To restore a openSUSE Leap 10-like behavior, M_MMAP_THRESHOLD should be set to 128*1024. This can be done with mallopt() call from the application, or via setting MALLOC_MMAP_THRESHOLD environment variable before running the application.

Kernel memory that is reclaimable (caches, described above) will be trimmed automatically during memory shortages. Most other kernel memory cannot be easily reduced but is a property of the workload given to the kernel.

Reducing the requirements of the user space workload will reduce the kernel memory usage (fewer processes, fewer open files and sockets, etc.)

If the memory cgroups feature is not needed, it can be switched off by passing cgroup_disable=memory on the kernel command line, reducing memory consumption of the kernel a bit.

One important change in writeback behavior since openSUSE Leap 10 is that modification to file-backed mmap() memory is accounted immediately as dirty memory (and subject to writeback). Whereas previously it would only be subject to writeback after it was unmapped, upon an msync() system call, or under heavy memory pressure.

Some applications do not expect mmap modifications to be subject to such writeback behavior, and performance can be reduced. Berkeley DB (and applications using it) is one known example that can cause problems. Increasing writeback ratios and times can improve this type of slowdown.

dirty_background_ratio and dirty_ratio together determine the pagecache writeback behavior. If these values are increased, more dirty memory is kept in the system for a longer time. With more dirty memory allowed in the system, the chance to improve throughput by avoiding writeback I/O and to submitting more optimal I/O patterns increases. However, more dirty memory can either harm latency when memory needs to be reclaimed or at points of data integrity (“synchronization points”) when it needs to be written back to disk.

The system is required to limit what percentage of the system's memory contains file-backed data that needs writing to disk. This guarantees that the system can always allocate the necessary data structures to complete I/O. The maximum amount of memory that may be dirty and requires writing at any given time is controlled by vm.dirty_ratio (/proc/sys/vm/dirty_ratio). The defaults are:

SLE-11-SP3:     vm.dirty_ratio = 40
SLE-12:         vm.dirty_ratio = 20

The primary advantage of using the lower ratio in SUSE Linux Enterprise 12 is that page reclamation and allocation in low memory situations completes faster as there is a higher probability that old clean pages will be quickly found and discarded. The secondary advantage is that if all data on the system must be synchronized, then the time to complete the operation on SUSE Linux Enterprise 12 will be lower than SUSE Linux Enterprise 11 SP3 by default. Most workloads will not notice this change as data is synchronized with fsync() by the application or data is not dirtied quickly enough to hit the limits.

There are exceptions and if your application is affected by this, it will manifest as an unexpected stall during writes. To prove it is affected by dirty data rate limiting then monitor /proc/PID_OF_APPLICATION/stack and it will be observed that the application spends significant time in balance_dirty_pages_ratelimited. If this is observed and it is a problem, then increase the value of vm.dirty_ratio to 40 to restore the SUSE Linux Enterprise 11 SP3 behavior.

It is important to note that the overall I/O throughput is the same regardless of the setting. The only difference is the timing of when the I/O is queued.

This is an example of using dd to asynchronously write 30% of memory to disk which would happen to be affected by the change in vm.dirty_ratio:

root # MEMTOTAL_MBYTES=`free -m | grep Mem: | awk '{print $2}'`
root # sysctl vm.dirty_ratio=40
root # dd if=/dev/zero of=zerofile ibs=1048576 count=$((MEMTOTAL_MBYTES*30/100))
2507145216 bytes (2.5 GB) copied, 8.00153 s, 313 MB/s
root # sysctl vm.dirty_ratio=20
dd if=/dev/zero of=zerofile ibs=1048576 count=$((MEMTOTAL_MBYTES*30/100))
2507145216 bytes (2.5 GB) copied, 10.1593 s, 247 MB/s

Note that the parameter affects the time it takes for the command to complete and the apparent write speed of the device. With dirty_ratio=40, more of the data is cached and written to disk in the background by the kernel. It is very important to note that the speed of I/O is identical in both cases. To demonstrate, this is the result when dd synchronizes the data before exiting:

root # sysctl vm.dirty_ratio=40
root # dd if=/dev/zero of=zerofile ibs=1048576 count=$((MEMTOTAL_MBYTES*30/100)) conv=fdatasync
2507145216 bytes (2.5 GB) copied, 21.0663 s, 119 MB/s
root # sysctl vm.dirty_ratio=20
root # dd if=/dev/zero of=zerofile ibs=1048576 count=$((MEMTOTAL_MBYTES*30/100)) conv=fdatasync
2507145216 bytes (2.5 GB) copied, 21.7286 s, 115 MB/s

Note that dirty_ratio had almost no impact here and is within the natural variability of a command. Hence, dirty_ratio does not directly impact I/O performance but it may affect the apparent performance of a workload that writes data asynchronously without synchronizing.

For the complete list of the VM tunable parameters, see /usr/src/linux/Documentation/sysctl/vm.txt (available after having installed the kernel-source package).