diff --git a/Documentation/vm/index.rst b/Documentation/vm/index.rst index eff5fbd492d08..c353b3f559244 100644 --- a/Documentation/vm/index.rst +++ b/Documentation/vm/index.rst @@ -17,6 +17,7 @@ various features of the Linux memory management swap_numa zswap + multigen_lru Kernel developers MM documentation ================================== diff --git a/Documentation/vm/multigen_lru.rst b/Documentation/vm/multigen_lru.rst new file mode 100644 index 0000000000000..adedff5319d9d --- /dev/null +++ b/Documentation/vm/multigen_lru.rst @@ -0,0 +1,134 @@ +.. SPDX-License-Identifier: GPL-2.0 + +===================== +Multigenerational LRU +===================== + +Quick Start +=========== +Build Configurations +-------------------- +:Required: Set ``CONFIG_LRU_GEN=y``. + +:Optional: Set ``CONFIG_LRU_GEN_ENABLED=y`` to turn the feature on by + default. + +Runtime Configurations +---------------------- +:Required: Write ``1`` to ``/sys/kernel/mm/lru_gen/enable`` if the + feature was not turned on by default. + +:Optional: Write ``N`` to ``/sys/kernel/mm/lru_gen/min_ttl_ms`` to + protect the working set of ``N`` milliseconds. The OOM killer is + invoked if this working set cannot be kept in memory. + +:Optional: Read ``/sys/kernel/debug/lru_gen`` to confirm the feature + is turned on. This file has the following output: + +:: + + memcg memcg_id memcg_path + node node_id + min_gen birth_time anon_size file_size + ... + max_gen birth_time anon_size file_size + +``min_gen`` is the oldest generation number and ``max_gen`` is the +youngest generation number. ``birth_time`` is in milliseconds. +``anon_size`` and ``file_size`` are in pages. + +Phones/Laptops/Workstations +--------------------------- +No additional configurations required. + +Servers/Data Centers +-------------------- +:To support more generations: Change ``CONFIG_NR_LRU_GENS`` to a + larger number. + +:To support more tiers: Change ``CONFIG_TIERS_PER_GEN`` to a larger + number. + +:To support full stats: Set ``CONFIG_LRU_GEN_STATS=y``. + +:Working set estimation: Write ``+ memcg_id node_id max_gen + [swappiness]`` to ``/sys/kernel/debug/lru_gen`` to invoke the aging, + which scans PTEs for accessed pages and then creates the next + generation ``max_gen+1``. A swap file and a non-zero ``swappiness``, + which overrides ``vm.swappiness``, are required to scan PTEs mapping + anon pages. + +:Proactive reclaim: Write ``- memcg_id node_id min_gen [swappiness] + [nr_to_reclaim]`` to ``/sys/kernel/debug/lru_gen`` to invoke the + eviction, which evicts generations less than or equal to ``min_gen``. + ``min_gen`` should be less than ``max_gen-1`` as ``max_gen`` and + ``max_gen-1`` are not fully aged and therefore cannot be evicted. + ``nr_to_reclaim`` can be used to limit the number of pages to evict. + Multiple command lines are supported, so does concatenation with + delimiters ``,`` and ``;``. + +Framework +========= +For each ``lruvec``, evictable pages are divided into multiple +generations. The youngest generation number is stored in +``lrugen->max_seq`` for both anon and file types as they are aged on +an equal footing. The oldest generation numbers are stored in +``lrugen->min_seq[2]`` separately for anon and file types as clean +file pages can be evicted regardless of swap and writeback +constraints. These three variables are monotonically increasing. +Generation numbers are truncated into +``order_base_2(CONFIG_NR_LRU_GENS+1)`` bits in order to fit into +``page->flags``. The sliding window technique is used to prevent +truncated generation numbers from overlapping. Each truncated +generation number is an index to an array of per-type and per-zone +lists ``lrugen->lists``. + +Each generation is then divided into multiple tiers. Tiers represent +levels of usage from file descriptors only. Pages accessed ``N`` times +via file descriptors belong to tier ``order_base_2(N)``. Each +generation contains at most ``CONFIG_TIERS_PER_GEN`` tiers, and they +require additional ``CONFIG_TIERS_PER_GEN-2`` bits in ``page->flags``. +In contrast to moving across generations which requires list +operations, moving across tiers only involves operations on +``page->flags`` and therefore has a negligible cost. A feedback loop +modeled after the PID controller monitors refault rates of all tiers +and decides when to protect pages from which tiers. + +The framework comprises two conceptually independent components: the +aging and the eviction, which can be invoked separately from user +space for the purpose of working set estimation and proactive reclaim. + +Aging +----- +The aging produces young generations. Given an ``lruvec``, the aging +traverses ``lruvec_memcg()->mm_list`` and calls ``walk_page_range()`` +to scan PTEs for accessed pages (a ``mm_struct`` list is maintained +for each ``memcg``). Upon finding one, the aging updates its +generation number to ``max_seq`` (modulo ``CONFIG_NR_LRU_GENS``). +After each round of traversal, the aging increments ``max_seq``. The +aging is due when both ``min_seq[2]`` have caught up with +``max_seq-1``. + +Eviction +-------- +The eviction consumes old generations. Given an ``lruvec``, the +eviction scans pages on the per-zone lists indexed by anon and file +``min_seq[2]`` (modulo ``CONFIG_NR_LRU_GENS``). It first tries to +select a type based on the values of ``min_seq[2]``. If they are +equal, it selects the type that has a lower refault rate. The eviction +sorts a page according to its updated generation number if the aging +has found this page accessed. It also moves a page to the next +generation if this page is from an upper tier that has a higher +refault rate than the base tier. The eviction increments +``min_seq[2]`` of a selected type when it finds all the per-zone lists +indexed by ``min_seq[2]`` of this selected type are empty. + +To-do List +========== +KVM Optimization +---------------- +Support shadow page table walk. + +NUMA Optimization +----------------- +Optimize page table walk for NUMA.