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diff --git a/Documentation/admin-guide/mm/concepts.rst b/Documentation/admin-guide/mm/concepts.rst new file mode 100644 index 000000000..fa0974fbe --- /dev/null +++ b/Documentation/admin-guide/mm/concepts.rst @@ -0,0 +1,223 @@ +.. _mm_concepts: + +================= +Concepts overview +================= + +The memory management in Linux is a complex system that evolved over the +years and included more and more functionality to support a variety of +systems from MMU-less microcontrollers to supercomputers. The memory +management for systems without an MMU is called ``nommu`` and it +definitely deserves a dedicated document, which hopefully will be +eventually written. Yet, although some of the concepts are the same, +here we assume that an MMU is available and a CPU can translate a virtual +address to a physical address. + +.. contents:: :local: + +Virtual Memory Primer +===================== + +The physical memory in a computer system is a limited resource and +even for systems that support memory hotplug there is a hard limit on +the amount of memory that can be installed. The physical memory is not +necessarily contiguous; it might be accessible as a set of distinct +address ranges. Besides, different CPU architectures, and even +different implementations of the same architecture have different views +of how these address ranges are defined. + +All this makes dealing directly with physical memory quite complex and +to avoid this complexity a concept of virtual memory was developed. + +The virtual memory abstracts the details of physical memory from the +application software, allows to keep only needed information in the +physical memory (demand paging) and provides a mechanism for the +protection and controlled sharing of data between processes. + +With virtual memory, each and every memory access uses a virtual +address. When the CPU decodes an instruction that reads (or +writes) from (or to) the system memory, it translates the `virtual` +address encoded in that instruction to a `physical` address that the +memory controller can understand. + +The physical system memory is divided into page frames, or pages. The +size of each page is architecture specific. Some architectures allow +selection of the page size from several supported values; this +selection is performed at the kernel build time by setting an +appropriate kernel configuration option. + +Each physical memory page can be mapped as one or more virtual +pages. These mappings are described by page tables that allow +translation from a virtual address used by programs to the physical +memory address. The page tables are organized hierarchically. + +The tables at the lowest level of the hierarchy contain physical +addresses of actual pages used by the software. The tables at higher +levels contain physical addresses of the pages belonging to the lower +levels. The pointer to the top level page table resides in a +register. When the CPU performs the address translation, it uses this +register to access the top level page table. The high bits of the +virtual address are used to index an entry in the top level page +table. That entry is then used to access the next level in the +hierarchy with the next bits of the virtual address as the index to +that level page table. The lowest bits in the virtual address define +the offset inside the actual page. + +Huge Pages +========== + +The address translation requires several memory accesses and memory +accesses are slow relatively to CPU speed. To avoid spending precious +processor cycles on the address translation, CPUs maintain a cache of +such translations called Translation Lookaside Buffer (or +TLB). Usually TLB is pretty scarce resource and applications with +large memory working set will experience performance hit because of +TLB misses. + +Many modern CPU architectures allow mapping of the memory pages +directly by the higher levels in the page table. For instance, on x86, +it is possible to map 2M and even 1G pages using entries in the second +and the third level page tables. In Linux such pages are called +`huge`. Usage of huge pages significantly reduces pressure on TLB, +improves TLB hit-rate and thus improves overall system performance. + +There are two mechanisms in Linux that enable mapping of the physical +memory with the huge pages. The first one is `HugeTLB filesystem`, or +hugetlbfs. It is a pseudo filesystem that uses RAM as its backing +store. For the files created in this filesystem the data resides in +the memory and mapped using huge pages. The hugetlbfs is described at +:ref:`Documentation/admin-guide/mm/hugetlbpage.rst <hugetlbpage>`. + +Another, more recent, mechanism that enables use of the huge pages is +called `Transparent HugePages`, or THP. Unlike the hugetlbfs that +requires users and/or system administrators to configure what parts of +the system memory should and can be mapped by the huge pages, THP +manages such mappings transparently to the user and hence the +name. See +:ref:`Documentation/admin-guide/mm/transhuge.rst <admin_guide_transhuge>` +for more details about THP. + +Zones +===== + +Often hardware poses restrictions on how different physical memory +ranges can be accessed. In some cases, devices cannot perform DMA to +all the addressable memory. In other cases, the size of the physical +memory exceeds the maximal addressable size of virtual memory and +special actions are required to access portions of the memory. Linux +groups memory pages into `zones` according to their possible +usage. For example, ZONE_DMA will contain memory that can be used by +devices for DMA, ZONE_HIGHMEM will contain memory that is not +permanently mapped into kernel's address space and ZONE_NORMAL will +contain normally addressed pages. + +The actual layout of the memory zones is hardware dependent as not all +architectures define all zones, and requirements for DMA are different +for different platforms. + +Nodes +===== + +Many multi-processor machines are NUMA - Non-Uniform Memory Access - +systems. In such systems the memory is arranged into banks that have +different access latency depending on the "distance" from the +processor. Each bank is referred to as a `node` and for each node Linux +constructs an independent memory management subsystem. A node has its +own set of zones, lists of free and used pages and various statistics +counters. You can find more details about NUMA in +:ref:`Documentation/vm/numa.rst <numa>` and in +:ref:`Documentation/admin-guide/mm/numa_memory_policy.rst <numa_memory_policy>`. + +Page cache +========== + +The physical memory is volatile and the common case for getting data +into the memory is to read it from files. Whenever a file is read, the +data is put into the `page cache` to avoid expensive disk access on +the subsequent reads. Similarly, when one writes to a file, the data +is placed in the page cache and eventually gets into the backing +storage device. The written pages are marked as `dirty` and when Linux +decides to reuse them for other purposes, it makes sure to synchronize +the file contents on the device with the updated data. + +Anonymous Memory +================ + +The `anonymous memory` or `anonymous mappings` represent memory that +is not backed by a filesystem. Such mappings are implicitly created +for program's stack and heap or by explicit calls to mmap(2) system +call. Usually, the anonymous mappings only define virtual memory areas +that the program is allowed to access. The read accesses will result +in creation of a page table entry that references a special physical +page filled with zeroes. When the program performs a write, a regular +physical page will be allocated to hold the written data. The page +will be marked dirty and if the kernel decides to repurpose it, +the dirty page will be swapped out. + +Reclaim +======= + +Throughout the system lifetime, a physical page can be used for storing +different types of data. It can be kernel internal data structures, +DMA'able buffers for device drivers use, data read from a filesystem, +memory allocated by user space processes etc. + +Depending on the page usage it is treated differently by the Linux +memory management. The pages that can be freed at any time, either +because they cache the data available elsewhere, for instance, on a +hard disk, or because they can be swapped out, again, to the hard +disk, are called `reclaimable`. The most notable categories of the +reclaimable pages are page cache and anonymous memory. + +In most cases, the pages holding internal kernel data and used as DMA +buffers cannot be repurposed, and they remain pinned until freed by +their user. Such pages are called `unreclaimable`. However, in certain +circumstances, even pages occupied with kernel data structures can be +reclaimed. For instance, in-memory caches of filesystem metadata can +be re-read from the storage device and therefore it is possible to +discard them from the main memory when system is under memory +pressure. + +The process of freeing the reclaimable physical memory pages and +repurposing them is called (surprise!) `reclaim`. Linux can reclaim +pages either asynchronously or synchronously, depending on the state +of the system. When the system is not loaded, most of the memory is free +and allocation requests will be satisfied immediately from the free +pages supply. As the load increases, the amount of the free pages goes +down and when it reaches a certain threshold (high watermark), an +allocation request will awaken the ``kswapd`` daemon. It will +asynchronously scan memory pages and either just free them if the data +they contain is available elsewhere, or evict to the backing storage +device (remember those dirty pages?). As memory usage increases even +more and reaches another threshold - min watermark - an allocation +will trigger `direct reclaim`. In this case allocation is stalled +until enough memory pages are reclaimed to satisfy the request. + +Compaction +========== + +As the system runs, tasks allocate and free the memory and it becomes +fragmented. Although with virtual memory it is possible to present +scattered physical pages as virtually contiguous range, sometimes it is +necessary to allocate large physically contiguous memory areas. Such +need may arise, for instance, when a device driver requires a large +buffer for DMA, or when THP allocates a huge page. Memory `compaction` +addresses the fragmentation issue. This mechanism moves occupied pages +from the lower part of a memory zone to free pages in the upper part +of the zone. When a compaction scan is finished free pages are grouped +together at the beginning of the zone and allocations of large +physically contiguous areas become possible. + +Like reclaim, the compaction may happen asynchronously in the ``kcompactd`` +daemon or synchronously as a result of a memory allocation request. + +OOM killer +========== + +It is possible that on a loaded machine memory will be exhausted and the +kernel will be unable to reclaim enough memory to continue to operate. In +order to save the rest of the system, it invokes the `OOM killer`. + +The `OOM killer` selects a task to sacrifice for the sake of the overall +system health. The selected task is killed in a hope that after it exits +enough memory will be freed to continue normal operation. |