The multi-gen LRU is an alternative LRU implementation that optimizes page reclaim and improves performance under memory pressure. Page reclaim decides the kernel's caching policy and ability to overcommit memory. It directly impacts the kswapd CPU usage and RAM efficiency.
Build the kernel with the following configurations.
/sys/kernel/mm/lru_gen/ contains stable ABIs described in the
enabled accepts different values to enable or disable the
following components. Its default value depends on
CONFIG_LRU_GEN_ENABLED. All the components should be enabled
unless some of them have unforeseen side effects. Writing to
enabled has no effect when a component is not supported by the
hardware, and valid values will be accepted even when the main switch
The main switch for the multi-gen LRU.
Clearing the accessed bit in leaf page table entries in large batches, when MMU sets it (e.g., on x86). This behavior can theoretically worsen lock contention (mmap_lock). If it is disabled, the multi-gen LRU will suffer a minor performance degradation for workloads that contiguously map hot pages, whose accessed bits can be otherwise cleared by fewer larger batches.
Clearing the accessed bit in non-leaf page table entries as well, when MMU sets it (e.g., on x86). This behavior was not verified on x86 varieties other than Intel and AMD. If it is disabled, the multi-gen LRU will suffer a negligible performance degradation.
Apply to all the components above.
echo y >/sys/kernel/mm/lru_gen/enabled cat /sys/kernel/mm/lru_gen/enabled 0x0007 echo 5 >/sys/kernel/mm/lru_gen/enabled cat /sys/kernel/mm/lru_gen/enabled 0x0005
Personal computers are more sensitive to thrashing because it can
cause janks (lags when rendering UI) and negatively impact user
experience. The multi-gen LRU offers thrashing prevention to the
majority of laptop and desktop users who do not have
Users can write
min_ttl_ms to prevent the working set of
N milliseconds from getting evicted. The OOM killer is triggered
if this working set cannot be kept in memory. In other words, this
option works as an adjustable pressure relief valve, and when open, it
terminates applications that are hopefully not being used.
Based on the average human detectable lag (~100ms),
eliminates intolerable janks due to thrashing. Larger values like
N=3000 make janks less noticeable at the risk of premature OOM
The default value
0 means disabled.
/sys/kernel/debug/lru_gen accepts commands described in the
following subsections. Multiple command lines are supported, so does
concatenation with delimiters
/sys/kernel/debug/lru_gen_full provides additional stats for
CONFIG_LRU_GEN_STATS=y keeps historical stats from
evicted generations in this file.
Working set estimation¶
Working set estimation measures how much memory an application needs in a given time interval, and it is usually done with little impact on the performance of the application. E.g., data centers want to optimize job scheduling (bin packing) to improve memory utilizations. When a new job comes in, the job scheduler needs to find out whether each server it manages can allocate a certain amount of memory for this new job before it can pick a candidate. To do so, the job scheduler needs to estimate the working sets of the existing jobs.
When it is read,
lru_gen returns a histogram of numbers of pages
accessed over different time intervals for each memcg and node.
MAX_NR_GENS decides the number of bins for each histogram. The
histograms are noncumulative.
memcg memcg_id memcg_path node node_id min_gen_nr age_in_ms nr_anon_pages nr_file_pages ... max_gen_nr age_in_ms nr_anon_pages nr_file_pages
Each bin contains an estimated number of pages that have been accessed
min_gen_nr contains the coldest pages
max_gen_nr contains the hottest pages, since
the former is the largest and that of the latter is the smallest.
Users can write the following command to
lru_gen to create a new
+ memcg_id node_id max_gen_nr [can_swap [force_scan]]
can_swap defaults to the swap setting and, if it is set to
it forces the scan of anon pages when swap is off, and vice versa.
force_scan defaults to
1 and, if it is set to
employs heuristics to reduce the overhead, which is likely to reduce
the coverage as well.
A typical use case is that a job scheduler runs this command at a certain time interval to create new generations, and it ranks the servers it manages based on the sizes of their cold pages defined by this time interval.
Proactive reclaim induces page reclaim when there is no memory pressure. It usually targets cold pages only. E.g., when a new job comes in, the job scheduler wants to proactively reclaim cold pages on the server it selected, to improve the chance of successfully landing this new job.
Users can write the following command to
lru_gen to evict
generations less than or equal to
- memcg_id node_id min_gen_nr [swappiness [nr_to_reclaim]]
min_gen_nr should be less than
max_gen_nr-1 are not fully aged (equivalent to
the active list) and therefore cannot be evicted.
overrides the default value in
nr_to_reclaim limits the number of pages to evict.
A typical use case is that a job scheduler runs this command before it tries to land a new job on a server. If it fails to materialize enough cold pages because of the overestimation, it retries on the next server according to the ranking result obtained from the working set estimation step. This less forceful approach limits the impacts on the existing jobs.