Memory Management – The Buddy Allocator

Every skilled professional should have an understanding of how the kernel does allocate memory: sooner or later it happens to get a system stuck complaining that it cannot allocate memory, despite the output of the "free" command showing that there's plenty of memory. This can get people confused, however this only means that the system cannot allocate kernel memory, despite it can still allocate system memory. The aim of this post is to clarify how the kernel allocates physical memory using the buddy algorithm, along with the available tools to check the state of physical memory.

The Buddy Memory Allocation Algorithm

Probably one of the most detailed explanations of the buddy algorithm is the one provided by Donald E. Knuth in the first volume ("Fundamental Algorithms") of "The Art of Computer Programming".

Note however that the buddy allocator used by Linux is a little different from the one depicted there.

Buddy allocator is a simple and cost effective memory allocation algorithm that leverages onto a binary tree that represents used or unused split memory blocks: the only notable drawback it has is that it does not completely avoid external fragmentation: although the coalesce of blocks tried when deallocating mitigate this problem a lot , it does not completely address it.

Initialization

In a buddy system, the entire memory space available for allocation is initially split into contiguous blocks; their size is calculated using the following formula:

PAGE_SIZE * 2 <sup>MAX_ORDER - 1</sup>

Linux defines at compile time 11 orders (see "include/linux/mmzone.h") and the default page size is 4 KiB.
This means that the size of blocks of order 11 (the biggest ones) is 4 KiB * 2¹⁰ = 4 MiB.

So for example in a system equipped with with 2 GiB of RAM at boot has a little few than 512 order 11 companion buddies:

2 GiB / 4 MiB = 2.048 MiB / 4 MiB = 512

they are not actually exactly 512 because of Memory Zones: for example, considering a x86_64 bit system, we have the first 16 MiB reserved to DMA Zone, whereas all the rest is DMA32 Zone.

So the actual number of companion buddies is:

( 2.048MiB - 16 MiB ) / 4 MiB = 2.032 MiB / 4 MiB = 508

Memory allocation

After this initial setup, at each page frame allocation request, a set of contiguous blocks of the order as close as possible to the request that can hold the requested size is allocated: the number of blocks is obviously calculated by dividing the requested size by the size of the block; if there's a remainder, an additional block is taken.

It's on the remainder that the buddy algorithm is focused: here it is started a recursive algorithm that splits this last block into two blocks of the lower order - those blocks are said companion buddies.

If the remainder is less than the lowest of the two companion buddies, the algorithm is recursively applied to it, otherwise the lowest companion buddy is allocated and it is calculated the difference from the remainder and the size of the other companion buddy, considering the result as a residual reminder. Then the algorithm is recursively applied to the unallocated companion buddy.

The recursion goes on until either the remainder becomes 0 or the order 0 is reached.

Memory de-allocation

When a process terminates, the blocks that were allocated to it are freed: the algorithm tries to merge a buddy that has become free with its companion buddy - this process is called coalescing. Remember that two blocks are said to be companion buddies if they are the outcome of a previous split of the same direct parent block.

Checking memory blocks availability

As we saw, on Linux system there are 11 orders, so the available blocks size is as depicted by the following table:

You can check the availability of blocks of any orders by inspecting /proc/buddyinfo file:

cat /proc/buddyinfo

an example output is as follows:

Node 0, zone      DMA      2      1      0      0      2      1      1      0      1      1      3
Node 0, zone    DMA32    386   1282   7088   3123    305     36      8      0      0      0      0
Node 0, zone   Normal  26930   4168   2670   1796   1021    456    123     11      1      0      0
Node 1, zone   Normal  37472   9367   8650   5344   3117   1471    515    147     20      0      0

from this output we can guess that:

• this is a x86_64 Kernel - there's no Highmem Zone
• this server has two physical CPUs (Node 0, Node 1)
• memory allocation is quite fragmented - both node 0 and 1 has exhausted< order above 9 (we start counting by 0) - this means that we have no more buddy companions of 2^(10-1) * PAGESIZE (512 * 4 Kib = 2 MiB) neither 2^(11-1) * PAGESIZE (1024 * 4 Kib = 4 MiB)

Memory Fragmentation

It occurs when while deallocating memory it is not possible to coalesce two companion buddies into one of the higher order: since the Linux kernel supports virtual memory management, most of the time physical memory fragmentation is not an issue, but anyway on systems that are up for a very long time it can become very difficult to allocate contiguous physical memory.

Running Out Of Memory

Let see how does memory fragmentation occur in practice: to keep things simple, consider the following example of a very small memory of 32K with a maximum order of 1.

• Process A requests 8K: a block of 8K (order 1) is allocated
• Process B requests 4K:  a block of 8K (order 1) is split into two blocks of 4K (order 0) and the lowest companion buddy is allocated to it
• Process C requests 12K:  a block of 8K (order 1) is allocated and another one is  split into two blocks of 4K (order 0) and the lowest companion buddy is allocated to it
• Process D requests 4K:  since there is already a buddy of 4K, companion of the one provided to B, the kernel assigns it to the process

We are getting close to the problem: when deleting B and C, only C  can coalesce two companion buddies to recreate the original 8K: B cannot do this since its companion is still used by D. This means that now we have two 8K (order1) and one 4K (order 0) free blocks.

Now suppose that process E is spawn, requesting 12K: it will get one 8K (order 1)  block: to keep memory contiguous, the remaining 4K are provided, splitting the last  8K (order 1)  block. So now we have two 4K (order 0) blocks.

You may think that you have still other 8K available for other processes, ... but this is not completely true: if process F is spawn, requesting 8K, you get the "cannot allocate memory error" and an errors like the following ones are logged:

May 10 18:03:22 [server] kernel: swapper: page allocation failure. order:1, mode:0x4020
May 10 20:41:16 [server] kernel: [process_e]: page allocation failure. order:1, mode:0xd0

despite you actually having 8K of available memory, they are the sum of two buddies of 4K that are not companions - and so they are not next to each other. Some memory allocators handle this exception requesting a pool of smaller blocks whose sum has the same size of the previous request, but if the requestor allocator cannot handle this exception then the application crashes.

The Linux Buddy Allocator

The buddy allocator used by Linux adds several enhancements to the algorithm we've just seen above - it adds the following extensions:

• Per-CPU pageset
• Partitions' buddy allocator
• Group by migration types

The per-CPU pageset is actually a way to reduce lock contention between processors and to optimize single page allocation

Group by migration types is an extension added to limit external memory fragmentation: this is a long-standing problem and several methods (and so several patches) have been submitted trying to deal with it. Some of them, although maybe controversial, have been actually merged.

One of the most important patches is probably the one from Mel Gorman, merged in Linux 5.0: it proved to be able to reduce memory fragmentation events by 94% on one-or-two-socket machines. One of Mel Gorman's merits is certainly having migrated the page recycling strategy from zone to node: the zone based recycling strategy was designed for 32-bit processors, but it was poorly performing on 64-bit processors.

As for migrations, the use of a virtual address space enable us to easily move data across physical pages:  only the value of the direct page table entry should be set to the new page frame number - the virtual address remains the same.
Problems arise when dealing with addresses belonging to the linear mapping area, ... things go really different: the virtual address equals the physical address plus the constant - this means that moving data across physical pages would actually modify the address and so any process that tries to access the old virtual address will fail. This means that migration of these pages should be avoided. This is the underlying reason for grouping by migration type: since managing the migration of both kinds of pages the same way can cause external memory fragmentation, the kernel implements different migration types and group pages on these while defragmenting.

The migration type is actually guided by the page allocation flag set when requesting for the page: for example, when dealing with file pages __GFP_RECLAIMABLE flag is set, whereas __GFP_MOVABLE is set for user space and so on

By inspecting /proc/pagetypeinfo we can view the distribution of each migration type:

cat /proc/pagetypeinfo

an example output is as follows:

Page block order: 9
Pages per block:  512

Free pages count per migrate type at order       0      1      2      3      4      5      6      7      8      9     10 
Node    0, zone      DMA, type    Unmovable      1      0      0      1      2      1      1      0      1      0      0 
Node    0, zone      DMA, type      Movable      0      0      0      0      0      0      0      0      0      1      3 
Node    0, zone      DMA, type  Reclaimable      0      0      0      0      0      0      0      0      0      0      0 
Node    0, zone      DMA, type   HighAtomic      0      0      0      0      0      0      0      0      0      0      0 
Node    0, zone      DMA, type      Isolate      0      0      0      0      0      0      0      0      0      0      0 
Node    0, zone    DMA32, type    Unmovable      0      0      0     24      5      0      0      1      1      1      0 
Node    0, zone    DMA32, type      Movable   6030   3386   1838   1145    686    337    139     50     19      2    544 
Node    0, zone    DMA32, type  Reclaimable      0      0      1      1      0      1      0      0      0      0      0 
Node    0, zone    DMA32, type   HighAtomic      0      0      0      0      0      0      0      0      0      0      0 
Node    0, zone    DMA32, type      Isolate      0      0      0      0      0      0      0      0      0      0      0 
Node    0, zone   Normal, type    Unmovable     44    132     87     33     11      2      1      1      1      0      0 
Node    0, zone   Normal, type      Movable      1      1     16      2      2      0      0      0      0      1      0 
Node    0, zone   Normal, type  Reclaimable      0      1     10      2      0      0      0      1      0      0      0 
Node    0, zone   Normal, type   HighAtomic      0      0      0      0      0      0      0      0      0      0      0 
Node    0, zone   Normal, type      Isolate      0      0      0      0      0      0      0      0      0      0      0 

Number of blocks type     Unmovable      Movable  Reclaimable   HighAtomic      Isolate 
Node 0, zone      DMA            1            7            0            0            0 
Node 0, zone    DMA32           50         1682           52            0            0 
Node 0, zone   Normal           83          158           15            0            0 

the above report groups pages by region (DMA, DMA32, Normal) and migration type. The available migration types are as follows:

When all of the pages of a given migration type are exhausted, the kernel - so to mitigate fragmentation - steals physical pages from other migration types, preferring pages with the largest block.

Take into account that it is always good to investigate too frequent page stealing: these events happen when external memory is actually fragmenting. To get these infos you should trace mm_page_alloc_extfrag kernel events and focus on the ones with fallback_order lower than pageblock_order - remember that on x86_64 platform pageblock_order is 9

Memory reclaiming And Compaction

Memory reclaiming is the process of freeing pages of Reclaimable migration type: it is quick and painless to free them, since their contents can be restored from disk blocks when needed. Each time two contiguous pages of the same order are reclaimed, they are merged into a page of the above order. This process is performed by the kswapd kernel thread.

Memory compaction is a more performance hitting process: it consists of seeking pages of MIGRATE_MOVABLE migration type from the bottom of the considered zone and, at the same time, scanning for free pages from the top of the zone; the process shifts the found pages of MIGRATE_MOVABLE migration type to the top: the straightforward outcome is increasing the number of the free pages on the bottom, and having all of the pages of MIGRATE_MOVABLE migration type on the top. This process, that was previously accomplished by kswapd, since kernel 4.6 is performed by a new dedicated kernel thread: kcompactd.

They can run asynchronous or in direct mode: when the low watermark is hit, they run asynchronously - that means that they are running since the system is anticipating forecast future memory allocation requests. If instead the min watermark is hit, they run in direct mode, ... that in turn means that they are forced to run each time a memory allocation request is made, trying to satisfy it. If the request still fails, the OOM killer is launched and a process gets killed.

These watermarks, expressed as pages, are bound to each memory zone, and are derived from the value of vm.min_free_kbytes using the following formulas:

• watermark[low] = watermark[min] * 5 / 4
• watermark[high] = watermark[min] * 3 / 2

Note that vm.min_free_kbytes is the sum of the mins of all of the zones. We can get the current values of both low and high watermarks for each of the zones as follows:

cat /proc/zoneinfo

for example, values for zone DMA32 are:

Node 0, zone DMA32
    ...
    pages free 403938
        min 11173
        low 13966
        high 16759
        ...

let's compare them to the outcome of the previous formulas:

• watermark[low] = watermark[min] * 5 / 4 = 11173 * 5 / 4 = 13966.25
• watermark[high] = watermark[min] * 3 / 2 = 11173 * 3 / 2 = 16759.5

the previous output shows also the watermark[min] of each zone. Note that it is measured in pages.

We can easily get the sum of watermark[min] of all of the zones:

cat /proc/zoneinfo | grep -A 1 "pages free"

the outcome is:

pages free     3977
        min      90
--
  pages free     403917
        min      11173
--
  pages free     0
        min      0
--
  pages free     0
        min      0
--
  pages free     0
        min      0

from the sum, we can guess the value of vm.min_free_kbytes:

(11173 + 90 ) * 4KiB = 45.052 KiB

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let's check it as follows:

cat /proc/sys/vm/min_free_kbytes

on my system the outcome is 45.052.

We can (carefully) play with when we are running out of physical memory: this alters the frequency of when kswapd and kompactd are woken up.

Despite the minimum value can be even 128 KB, you should be very careful while adjusting this setting: a value lower than 1'024 KiB will subtly break the system, making it prone to deadlock under high loads; increasing it too much will make kswapd start earlier than is actually required for recycling, recovering more memory (until the watermark[high] is reached). The outcome of reserving too much free memory is reducing memory available to applications, and this can lead to the OOM Killer running too frequently.

We can go by trials to figure out the proper value by issuing a command like the following one multiple times increasing the value at each run:

sysctl -w vm.min_free_kbytes=101376

once we get a suitable value, we must store it into a sysctl config file so to have it automatically set at boot:

echo "vm.min_free_kbytes=101376" >> /etc/sysctl.d/99-buddy-fix.conf

As we saw before, memory compaction is a more performance hitting process: besides the watermark[low], there's also a fragmentation specific threshold that should be hit before actually starting a compaction: it is set governed by the vm/extfrag_threshold tunable - memory compaction is triggered when the fragmentation index is less or equal to its value:

cat /proc/sys/vm/extfrag_threshold

on my system the output is

500

that by the way is the default value: we can of course adjust it by setting a different value, making it more or less prone to compact memory or direct reclaim to satisfy a high-order allocation.

We can check the fragmentation degree of each order by looking at the /sys/kernel/debug/extfrag/extfrag_index file:

sudo cat /sys/kernel/debug/extfrag/extfrag_index

if you issue this right after the system boot it looks as follows:

Node 0, zone DMA -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000
Node 0, zone DMA32 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000

as you can see, it reports the external fragmentation degree grouped by node for each of the orders. The meaning of these values is as follows:

• tending towards 0 imply allocations would fail due to lack of memory
• tending towards 1000 imply failures are due to fragmentation
• -1 implies that the allocation will succeed as long as watermarks are met