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Diffstat (limited to 'Documentation/admin-guide/mm/concepts.rst')
| -rw-r--r-- | Documentation/admin-guide/mm/concepts.rst | 51 |
1 files changed, 26 insertions, 25 deletions
diff --git a/Documentation/admin-guide/mm/concepts.rst b/Documentation/admin-guide/mm/concepts.rst index 291699c810d4..c2531b14bf46 100644 --- a/Documentation/admin-guide/mm/concepts.rst +++ b/Documentation/admin-guide/mm/concepts.rst @@ -4,13 +4,13 @@ Concepts overview ================= -The memory management in Linux is complex system that evolved over the -years and included more and more functionality to support variety of +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 MMU is called ``nommu`` and it +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 MMU is available and CPU can translate a virtual +here we assume that an MMU is available and a CPU can translate a virtual address to a physical address. .. contents:: :local: @@ -21,10 +21,10 @@ 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 -necessary contiguous, it might be accessible as a set of distinct +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 view -how these address ranges defined. +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. @@ -48,8 +48,8 @@ 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 virtual address used by programs to real address in -the physical memory. The page tables organized hierarchically. +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 @@ -121,8 +121,8 @@ 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 as `node` and for each node Linux -constructs an independent memory management subsystem. A node has it's +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 @@ -149,9 +149,9 @@ 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, regular +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 will decide to repurpose it, +will be marked dirty and if the kernel decides to repurpose it, the dirty page will be swapped out. Reclaim @@ -181,8 +181,8 @@ 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 system is not loaded, most of the memory is free -and allocation request will be satisfied immediately from the free +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 @@ -190,7 +190,7 @@ 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 the `direct reclaim`. In this case allocation is stalled +will trigger `direct reclaim`. In this case allocation is stalled until enough memory pages are reclaimed to satisfy the request. Compaction @@ -200,7 +200,7 @@ 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 large +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 @@ -208,15 +208,16 @@ 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 ``kcompactd`` -daemon or synchronously as a result of memory allocation request. +Like reclaim, the compaction may happen asynchronously in the ``kcompactd`` +daemon or synchronously as a result of a memory allocation request. OOM killer ========== -It may happen, that on a loaded machine memory will be exhausted. When -the kernel detects that the system runs out of memory (OOM) it invokes -`OOM killer`. Its mission is simple: all it has to do is to select 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. +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. |