From: SeongJae Park Date: Wed, 4 Jun 2025 18:31:27 +0000 (-0700) Subject: Docs/admin-guide/mm/damon: add DAMON_STAT usage document X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=7c33c6c47456c55ca043bb95a34968ef022bf625;p=thirdparty%2Fkernel%2Flinux.git Docs/admin-guide/mm/damon: add DAMON_STAT usage document Document DAMON_STAT usage and add a link to it on DAMON admin-guide page. Link: https://lkml.kernel.org/r/20250604183127.13968-5-sj@kernel.org Signed-off-by: SeongJae Park Cc: Jonathan Corbet Signed-off-by: Andrew Morton --- diff --git a/Documentation/admin-guide/mm/damon/index.rst b/Documentation/admin-guide/mm/damon/index.rst index bc7e976120e0b..3ce3164480c7b 100644 --- a/Documentation/admin-guide/mm/damon/index.rst +++ b/Documentation/admin-guide/mm/damon/index.rst @@ -14,3 +14,4 @@ access monitoring and access-aware system operations. usage reclaim lru_sort + stat diff --git a/Documentation/admin-guide/mm/damon/stat.rst b/Documentation/admin-guide/mm/damon/stat.rst new file mode 100644 index 0000000000000..4c517c2c219aa --- /dev/null +++ b/Documentation/admin-guide/mm/damon/stat.rst @@ -0,0 +1,69 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=================================== +Data Access Monitoring Results Stat +=================================== + +Data Access Monitoring Results Stat (DAMON_STAT) is a static kernel module that +is aimed to be used for simple access pattern monitoring. It monitors accesses +on the system's entire physical memory using DAMON, and provides simplified +access monitoring results statistics, namely idle time percentiles and +estimated memory bandwidth. + +Monitoring Accuracy and Overhead +================================ + +DAMON_STAT uses monitoring intervals :ref:`auto-tuning +` to make its accuracy high and +overhead minimum. It auto-tunes the intervals aiming 4 % of observable access +events to be captured in each snapshot, while limiting the resulting sampling +events to be 5 milliseconds in minimum and 10 seconds in maximum. On a few +production server systems, it resulted in consuming only 0.x % single CPU time, +while capturing reasonable quality of access patterns. + +Interface: Module Parameters +============================ + +To use this feature, you should first ensure your system is running on a kernel +that is built with ``CONFIG_DAMON_STAT=y``. The feature can be enabled by +default at build time, by setting ``CONFIG_DAMON_STAT_ENABLED_DEFAULT`` true. + +To let sysadmins enable or disable it at boot and/or runtime, and read the +monitoring results, DAMON_STAT provides module parameters. Following +sections are descriptions of the parameters. + +enabled +------- + +Enable or disable DAMON_STAT. + +You can enable DAMON_STAT by setting the value of this parameter as ``Y``. +Setting it as ``N`` disables DAMON_STAT. The default value is set by +``CONFIG_DAMON_STAT_ENABLED_DEFAULT`` build config option. + +estimated_memory_bandwidth +-------------------------- + +Estimated memory bandwidth consumption (bytes per second) of the system. + +DAMON_STAT reads observed access events on the current DAMON results snapshot +and converts it to memory bandwidth consumption estimation in bytes per second. +The resulting metric is exposed to user via this read-only parameter. Because +DAMON uses sampling, this is only an estimation of the access intensity rather +than accurate memory bandwidth. + +memory_idle_ms_percentiles +-------------------------- + +Per-byte idle time (milliseconds) percentiles of the system. + +DAMON_STAT calculates how long each byte of the memory was not accessed until +now (idle time), based on the current DAMON results snapshot. If DAMON found a +region of access frequency (nr_accesses) larger than zero, every byte of the +region gets zero idle time. If a region has zero access frequency +(nr_accesses), how long the region was keeping the zero access frequency (age) +becomes the idle time of every byte of the region. Then, DAMON_STAT exposes +the percentiles of the idle time values via this read-only parameter. Reading +the parameter returns 101 idle time values in milliseconds, separated by comma. +Each value represents 0-th, 1st, 2nd, 3rd, ..., 99th and 100th percentile idle +times.