1.. SPDX-License-Identifier: GPL-2.0 2 3============= 4Multi-Gen LRU 5============= 6The multi-gen LRU is an alternative LRU implementation that optimizes 7page reclaim and improves performance under memory pressure. Page 8reclaim decides the kernel's caching policy and ability to overcommit 9memory. It directly impacts the kswapd CPU usage and RAM efficiency. 10 11Quick start 12=========== 13Build the kernel with the following configurations. 14 15* ``CONFIG_LRU_GEN=y`` 16* ``CONFIG_LRU_GEN_ENABLED=y`` 17 18All set! 19 20Runtime options 21=============== 22``/sys/kernel/mm/lru_gen/`` contains stable ABIs described in the 23following subsections. 24 25Kill switch 26----------- 27``enabled`` accepts different values to enable or disable the 28following components. Its default value depends on 29``CONFIG_LRU_GEN_ENABLED``. All the components should be enabled 30unless some of them have unforeseen side effects. Writing to 31``enabled`` has no effect when a component is not supported by the 32hardware, and valid values will be accepted even when the main switch 33is off. 34 35====== =============================================================== 36Values Components 37====== =============================================================== 380x0001 The main switch for the multi-gen LRU. 390x0002 Clearing the accessed bit in leaf page table entries in large 40 batches, when MMU sets it (e.g., on x86). This behavior can 41 theoretically worsen lock contention (mmap_lock). If it is 42 disabled, the multi-gen LRU will suffer a minor performance 43 degradation for workloads that contiguously map hot pages, 44 whose accessed bits can be otherwise cleared by fewer larger 45 batches. 460x0004 Clearing the accessed bit in non-leaf page table entries as 47 well, when MMU sets it (e.g., on x86). This behavior was not 48 verified on x86 varieties other than Intel and AMD. If it is 49 disabled, the multi-gen LRU will suffer a negligible 50 performance degradation. 51[yYnN] Apply to all the components above. 52====== =============================================================== 53 54E.g., 55:: 56 57 echo y >/sys/kernel/mm/lru_gen/enabled 58 cat /sys/kernel/mm/lru_gen/enabled 59 0x0007 60 echo 5 >/sys/kernel/mm/lru_gen/enabled 61 cat /sys/kernel/mm/lru_gen/enabled 62 0x0005 63 64Thrashing prevention 65-------------------- 66Personal computers are more sensitive to thrashing because it can 67cause janks (lags when rendering UI) and negatively impact user 68experience. The multi-gen LRU offers thrashing prevention to the 69majority of laptop and desktop users who do not have ``oomd``. 70 71Users can write ``N`` to ``min_ttl_ms`` to prevent the working set of 72``N`` milliseconds from getting evicted. The OOM killer is triggered 73if this working set cannot be kept in memory. In other words, this 74option works as an adjustable pressure relief valve, and when open, it 75terminates applications that are hopefully not being used. 76 77Based on the average human detectable lag (~100ms), ``N=1000`` usually 78eliminates intolerable janks due to thrashing. Larger values like 79``N=3000`` make janks less noticeable at the risk of premature OOM 80kills. 81 82The default value ``0`` means disabled. 83 84Experimental features 85===================== 86``/sys/kernel/debug/lru_gen`` accepts commands described in the 87following subsections. Multiple command lines are supported, so does 88concatenation with delimiters ``,`` and ``;``. 89 90``/sys/kernel/debug/lru_gen_full`` provides additional stats for 91debugging. ``CONFIG_LRU_GEN_STATS=y`` keeps historical stats from 92evicted generations in this file. 93 94Working set estimation 95---------------------- 96Working set estimation measures how much memory an application needs 97in a given time interval, and it is usually done with little impact on 98the performance of the application. E.g., data centers want to 99optimize job scheduling (bin packing) to improve memory utilizations. 100When a new job comes in, the job scheduler needs to find out whether 101each server it manages can allocate a certain amount of memory for 102this new job before it can pick a candidate. To do so, the job 103scheduler needs to estimate the working sets of the existing jobs. 104 105When it is read, ``lru_gen`` returns a histogram of numbers of pages 106accessed over different time intervals for each memcg and node. 107``MAX_NR_GENS`` decides the number of bins for each histogram. The 108histograms are noncumulative. 109:: 110 111 memcg memcg_id memcg_path 112 node node_id 113 min_gen_nr age_in_ms nr_anon_pages nr_file_pages 114 ... 115 max_gen_nr age_in_ms nr_anon_pages nr_file_pages 116 117Each bin contains an estimated number of pages that have been accessed 118within ``age_in_ms``. E.g., ``min_gen_nr`` contains the coldest pages 119and ``max_gen_nr`` contains the hottest pages, since ``age_in_ms`` of 120the former is the largest and that of the latter is the smallest. 121 122Users can write the following command to ``lru_gen`` to create a new 123generation ``max_gen_nr+1``: 124 125 ``+ memcg_id node_id max_gen_nr [can_swap [force_scan]]`` 126 127``can_swap`` defaults to the swap setting and, if it is set to ``1``, 128it forces the scan of anon pages when swap is off, and vice versa. 129``force_scan`` defaults to ``1`` and, if it is set to ``0``, it 130employs heuristics to reduce the overhead, which is likely to reduce 131the coverage as well. 132 133A typical use case is that a job scheduler runs this command at a 134certain time interval to create new generations, and it ranks the 135servers it manages based on the sizes of their cold pages defined by 136this time interval. 137 138Proactive reclaim 139----------------- 140Proactive reclaim induces page reclaim when there is no memory 141pressure. It usually targets cold pages only. E.g., when a new job 142comes in, the job scheduler wants to proactively reclaim cold pages on 143the server it selected, to improve the chance of successfully landing 144this new job. 145 146Users can write the following command to ``lru_gen`` to evict 147generations less than or equal to ``min_gen_nr``. 148 149 ``- memcg_id node_id min_gen_nr [swappiness [nr_to_reclaim]]`` 150 151``min_gen_nr`` should be less than ``max_gen_nr-1``, since 152``max_gen_nr`` and ``max_gen_nr-1`` are not fully aged (equivalent to 153the active list) and therefore cannot be evicted. ``swappiness`` 154overrides the default value in ``/proc/sys/vm/swappiness``. 155``nr_to_reclaim`` limits the number of pages to evict. 156 157A typical use case is that a job scheduler runs this command before it 158tries to land a new job on a server. If it fails to materialize enough 159cold pages because of the overestimation, it retries on the next 160server according to the ranking result obtained from the working set 161estimation step. This less forceful approach limits the impacts on the 162existing jobs. 163