
Applies to SUSE Linux Enterprise Server 15

14 Tuning the Memory Management Subsystem Edit source

To understand and tune the memory management behavior of the kernel, it is important to first have an overview of how it works and cooperates with other subsystems.

The memory management subsystem, also called the virtual memory manager, will subsequently be called VM. The role of the VM is to manage the allocation of physical memory (RAM) for the entire kernel and user programs. It is also responsible for providing a virtual memory environment for user processes (managed via POSIX APIs with Linux extensions). Finally, the VM is responsible for freeing up RAM when there is a shortage, either by trimming caches or swapping out anonymous memory.

The most important thing to understand when examining and tuning VM is how its caches are managed. The basic goal of the VM's caches is to minimize the cost of I/O as generated by swapping and file system operations (including network file systems). This is achieved by avoiding I/O completely, or by submitting I/O in better patterns.

Free memory will be used and filled up by these caches as required. The more memory is available for caches and anonymous memory, the more effectively caches and swapping will operate. However, if a memory shortage is encountered, caches will be trimmed or memory will be swapped out.

For a particular workload, the first thing that can be done to improve performance is to increase memory and reduce the frequency that memory must be trimmed or swapped. The second thing is to change the way caches are managed by changing kernel parameters.

Finally, the workload itself should be examined and tuned as well. If an application is allowed to run more processes or threads, effectiveness of VM caches can be reduced, if each process is operating in its own area of the file system. Memory overheads are also increased. If applications allocate their own buffers or caches, larger caches will mean that less memory is available for VM caches. However, more processes and threads can mean more opportunity to overlap and pipeline I/O, and may take better advantage of multiple cores. Experimentation will be required for the best results.

14.1 Memory Usage Edit source

Memory allocations in general can be characterized as pinned (also known as unreclaimable), reclaimable or swappable.

14.1.1 Anonymous Memory Edit source

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.

14.1.2 Pagecache Edit source

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.

14.1.3 Buffercache Edit source

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.

14.1.4 Buffer Heads Edit source

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.

14.1.5 Writeback Edit source

As applications write to files, the pagecache becomes dirty and the buffercache may become dirty. When the amount of dirty memory reaches a specified number of pages in bytes (vm.dirty_background_bytes), or when the amount of dirty memory reaches a specific ratio to total memory (vm.dirty_background_ratio), or when the pages have been dirty for longer than a specified amount of time (vm.dirty_expire_centisecs), the kernel begins writeback of pages starting with files that had the pages dirtied first. The background bytes and ratios are mutually exclusive and setting one will overwrite the other. 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 or vm.dirty_ratio), then applications begin to be throttled to prevent dirty data exceeding this threshold.

14.1.6 Readahead Edit source

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).

14.1.7 VFS caches Edit source Inode Cache Edit source

This is an in-memory cache of the inode structures for each file system. These contain attributes such as the file size, permissions and ownership, and pointers to the file data. Directory Entry Cache Edit source

This is an in-memory cache of the directory entries in the system. These contain a name (the name of a file), the inode which it refers to, and children entries. This cache is used when traversing the directory structure and accessing a file by name.

14.2 Reducing Memory Usage Edit source

14.2.1 Reducing malloc (Anonymous) Usage Edit source

Applications running on SUSE Linux Enterprise Server 15 can allocate more memory compared to SUSE Linux Enterprise Server 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 SUSE Linux Enterprise Server 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.

14.2.2 Reducing Kernel Memory Overheads Edit source

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.)

14.2.3 Memory Controller (Memory Cgroups) Edit source

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. There is also a slight performance benefit as there is a small amount of accounting overhead when memory cgroups are available even if none are configured.

14.3 Virtual Memory Manager (VM) Tunable Parameters Edit source

When tuning the VM it should be understood that some changes will take time to affect the workload and take full effect. If the workload changes throughout the day, it may behave very differently at different times. A change that increases throughput under some conditions may decrease it under other conditions.

14.3.1 Reclaim Ratios Edit source


This control is used to define how aggressively the kernel swaps out anonymous memory relative to pagecache and other caches. Increasing the value increases the amount of swapping. The default value is 60.

Swap I/O tends to be much less efficient than other I/O. However, some pagecache pages will be accessed much more frequently than less used anonymous memory. The right balance should be found here.

If swap activity is observed during slowdowns, it may be worth reducing this parameter. If there is a lot of I/O activity and the amount of pagecache in the system is rather small, or if there are large dormant applications running, increasing this value might improve performance.

Note that the more data is swapped out, the longer the system will take to swap data back in when it is needed.


This variable controls the tendency of the kernel to reclaim the memory which is used for caching of VFS caches, versus pagecache and swap. Increasing this value increases the rate at which VFS caches are reclaimed.

It is difficult to know when this should be changed, other than by experimentation. The slabtop command (part of the package procps) shows top memory objects used by the kernel. The vfs caches are the "dentry" and the "*_inode_cache" objects. If these are consuming a large amount of memory in relation to pagecache, it may be worth trying to increase pressure. Could also help to reduce swapping. The default value is 100.


This controls the amount of memory that is kept free for use by special reserves including atomic allocations (those which cannot wait for reclaim). This should not normally be lowered unless the system is being very carefully tuned for memory usage (normally useful for embedded rather than server applications). If page allocation failure messages and stack traces are frequently seen in logs, min_free_kbytes could be increased until the errors disappear. There is no need for concern, if these messages are very infrequent. The default value depends on the amount of RAM.


Broadly speaking, free memory has high, low and min watermarks. When the low watermark is reached then kswapd wakes to reclaim memory in the background. It stays awake until free memory reaches the high watermark. Applications will stall and reclaim memory when the low watermark is reached.


Workloads that frequently stall in direct reclaim, accounted by allocstall in /proc/vmstat, may benefit from altering this parameter. Similarly, if kswapd is sleeping prematurely, as accounted for by kswapd_low_wmark_hit_quickly, then it may indicate that the number of pages kept free to avoid stalls is too low.

14.3.2 Writeback Parameters Edit source

One important change in writeback behavior since SUSE Linux Enterprise Server 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.


This is the percentage of the total amount of free and reclaimable memory. When the amount of dirty pagecache exceeds this percentage, writeback threads start writing back dirty memory. The default value is 10 (%).


This contains the amount of dirty memory at which the background kernel flusher threads will start writeback. dirty_background_bytes is the counterpart of dirty_background_ratio. If one of them is set, the other one will automatically be read as 0.


Similar percentage value as for dirty_background_ratio. When this is exceeded, applications that want to write to the pagecache are blocked and wait for kernel background flusher threads to reduce the amount of dirty memory. The default value is 20 (%).


This file controls the same tunable as dirty_ratio however the amount of dirty memory is in bytes as opposed to a percentage of reclaimable memory. Since both dirty_ratio and dirty_bytes control the same tunable, if one of them is set, the other one will automatically be read as 0. The minimum value allowed for dirty_bytes is two pages (in bytes); any value lower than this limit will be ignored and the old configuration will be retained.


Data which has been dirty in-memory for longer than this interval will be written out next time a flusher thread wakes up. Expiration is measured based on the modification time of a file's inode. Therefore, multiple dirtied pages from the same file will all be written when the interval is exceeded.

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.

14.3.3 Timing Differences of I/O Writes between SUSE Linux Enterprise 12 and SUSE Linux Enterprise 11 Edit source

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 can be dirty and requires writing at any 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
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
2507145216 bytes (2.5 GB) copied, 21.0663 s, 119 MB/s
root # sysctl vm.dirty_ratio=20
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.

14.3.4 Readahead Parameters Edit source


If one or more processes are sequentially reading a file, the kernel reads some data in advance (ahead) to reduce the amount of time that processes need to wait for data to be available. The actual amount of data being read in advance is computed dynamically, based on how much "sequential" the I/O seems to be. This parameter sets the maximum amount of data that the kernel reads ahead for a single file. If you observe that large sequential reads from a file are not fast enough, you can try increasing this value. Increasing it too far may result in readahead thrashing where pagecache used for readahead is reclaimed before it can be used, or slowdowns because of a large amount of useless I/O. The default value is 512 (KB).

14.3.5 Transparent Huge Page Parameters Edit source

Transparent Huge Pages (THP) provide a way to dynamically allocate huge pages either on‑demand by the process or deferring the allocation until later via the khugepaged kernel thread. This method is distinct from the use of hugetlbfs to manually manage their allocation and use. Workloads with contiguous memory access patterns can benefit greatly from THP. A 1000-fold decrease in page faults can be observed when running synthetic workloads with contiguous memory access patterns.

There are cases when THP may be undesirable. Workloads with sparse memory access patterns can perform poorly with THP due to excessive memory usage. For example, 2 MB of memory may be used at fault time instead of 4 KB for each fault and ultimately lead to premature page reclaim. On releases older than SUSE Linux Enterprise 12 SP2, it was possible for an application to stall for long periods of time trying to allocate a THP which frequently led to a recommendation of disabling THP. Such recommendations should be re-evaluated for SUSE Linux Enterprise 12 SP3

The behavior of THP may be configured via the transparent_hugepage= kernel parameter or via sysfs. For example, it may be disabled by adding the kernel parameter transparent_hugepage=never, rebuilding your grub2 configuration, and rebooting. Verify if THP is disabled with:

root # cat /sys/kernel/mm/transparent_hugepage/enabled
always madvise [never]

If disabled, the value never is shown in square brackets like in the example above. A value of always will always try and use THP at fault time but defer to khugepaged if the allocation fails. A value of madvise will only allocate THP for address spaces explicitly specified by an application.


This parameter controls how much effort an application commits when allocating a THP. A value of always is the default for SUSE Linux Enterprise 12 SP1 and earlier releases that supported THP. If a THP is not available, the application will try to defragment memory. It potentially incurs large stalls in an application if the memory is fragmented and a THP is not available.

A value of madvise means that THP allocation requests will only defragment if the application explicitly requests it. This is the default for SUSE Linux Enterprise 12 SP2 and later releases.

defer is only available on SUSE Linux Enterprise 12 SP2 and later releases. If a THP is not available, the application will fall back to using small pages if a THP is not available. It will wake the kswapd and kcompactd kernel threads to defragment memory in the background and a THP will be allocated later by khugepaged.

The final option never will use small pages if a THP is unavailable but no other action will take place.

14.3.6 khugepaged Parameters Edit source

khugepaged will be automatically started when transparent_hugepage is set to always or madvise, and it will be automatically shut down if it is set to never. Normally this runs at low frequency but the behavior can be tuned.


A value of 0 will disable khugepaged even though THP may still be used at fault time. This may be important for latency-sensitive applications that benefit from THP but cannot tolerate a stall if khugepaged tries to update an application memory usage.


This parameter controls how many pages are scanned by khugepaged in a single pass. A scan identifies small pages that can be reallocated as THP. Increasing this value will allocate THP in the background faster at the cost of CPU usage.


khugepaged sleeps for a short interval specified by this parameter after each pass to limit how much CPU usage is used. Reducing this value will allocate THP in the background faster at the cost of CPU usage. A value of 0 will force continual scanning.


This parameter controls how long khugepaged will sleep in the event it fails to allocate a THP in the background waiting for kswapd and kcompactd to take action.

The remaining parameters for khugepaged are rarely useful for performance tuning but are fully documented in /usr/src/linux/Documentation/vm/transhuge.txt

14.3.7 Further VM Parameters Edit source

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).

14.4 Monitoring VM Behavior Edit source

Some simple tools that can help monitor VM behavior:

  1. vmstat: This tool gives a good overview of what the VM is doing. See Section 2.1.1, “vmstat for details.

  2. /proc/meminfo: This file gives a detailed breakdown of where memory is being used. See Section 2.4.2, “Detailed Memory Usage: /proc/meminfo for details.

  3. slabtop: This tool provides detailed information about kernel slab memory usage. buffer_head, dentry, inode_cache, ext3_inode_cache, etc. are the major caches. This command is available with the package procps.

  4. /proc/vmstat: This file gives a detailed breakdown of internal VM behavior. The information contained within is implementation specific and may not always be available. Some information is duplicated in /proc/meminfo and other information can be presented in a friendly fashion by utilities. For maximum utility, this file needs to be monitored over time to observe rates of change. The most important pieces of information that are hard to derive from other sources are as follows:

    pgscan_kswapd_*, pgsteal_kswapd_*

    These report respectively the number of pages scanned and reclaimed by kswapd since the system started. The ratio between these values can be interpreted as the reclaim efficiency with a low efficiency implying that the system is struggling to reclaim memory and may be thrashing. Light activity here is generally not something to be concerned with.

    pgscan_direct_*, pgsteal_direct_*

    These report respectively the number of pages scanned and reclaimed by an application directly. This is correlated with increases in the allocstall counter. This is more serious than kswapd activity as these events indicate that processes are stalling. Heavy activity here combined with kswapd and high rates of pgpgin, pgpout and/or high rates of pswapin or pswpout are signs that a system is thrashing heavily.

    More detailed information can be obtained using tracepoints.

    thp_fault_alloc, thp_fault_fallback

    These counters correspond to how many THPs were allocated directly by an application and how many times a THP was not available and small pages were used. Generally a high fallback rate is harmless unless the application is very sensitive to TLB pressure.

    thp_collapse_alloc, thp_collapse_alloc_failed

    These counters correspond to how many THPs were allocated by khugepaged and how many times a THP was not available and small pages were used. A high fallback rate implies that the system is fragmented and THPs are not being used even when the memory usage by applications would allow them. It is only a problem for applications that are sensitive to TLB pressure.

    compact_*_scanned, compact_stall, compact_fail, compact_success

    These counters may increase when THP is enabled and the system is fragmented. compact_stall is incremented when an application stalls allocating THP. The remaining counters account for pages scanned, the number of defragmentation events that succeeded or failed.

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