* [PATCH v5 1/4] mm/ksm: add ksm advisor
2023-12-18 23:10 [PATCH v5 0/4] mm/ksm: Add ksm advisor Stefan Roesch
@ 2023-12-18 23:10 ` Stefan Roesch
2023-12-18 23:10 ` [PATCH v5 2/4] mm/ksm: add sysfs knobs for advisor Stefan Roesch
` (2 subsequent siblings)
3 siblings, 0 replies; 6+ messages in thread
From: Stefan Roesch @ 2023-12-18 23:10 UTC (permalink / raw)
To: kernel-team; +Cc: shr, akpm, david, hannes, riel, linux-kernel, linux-mm
This adds the ksm advisor. The ksm advisor automatically manages the
pages_to_scan setting to achieve a target scan time. The target scan
time defines how many seconds it should take to scan all the candidate
KSM pages. In other words the pages_to_scan rate is changed by the
advisor to achieve the target scan time. The algorithm has a max and min
value to:
- guarantee responsiveness to changes
- limit CPU resource consumption
The respective parameters are:
- ksm_advisor_target_scan_time (how many seconds a scan should take)
- ksm_advisor_max_cpu (maximum value for cpu percent usage)
- ksm_advisor_min_pages (minimum value for pages_to_scan per batch)
- ksm_advisor_max_pages (maximum value for pages_to_scan per batch)
The algorithm calculates the change value based on the target scan time
and the previous scan time. To avoid pertubations an exponentially
weighted moving average is applied.
The advisor is managed by two main parameters: target scan time,
cpu max time for the ksmd background thread. These parameters determine
how aggresive ksmd scans.
In addition there are min and max values for the pages_to_scan parameter
to make sure that its initial and max values are not set too low or too
high. This ensures that it is able to react to changes quickly enough.
The default values are:
- target scan time: 200 secs
- max cpu: 70%
- min pages: 500
- max pages: 30000
By default the advisor is disabled. Currently there are two advisors:
none and scan-time.
Tests with various workloads have shown considerable CPU savings. Most
of the workloads I have investigated have more candidate pages during
startup, once the workload is stable in terms of memory, the number of
candidate pages is reduced. Without the advisor, the pages_to_scan needs
to be sized for the maximum number of candidate pages. So having this
advisor definitely helps in reducing CPU consumption.
For the instagram workload, the advisor achieves a 25% CPU reduction.
Once the memory is stable, the pages_to_scan parameter gets reduced to
about 40% of its max value.
Signed-off-by: Stefan Roesch <shr@devkernel.io>
Acked-by: David Hildenbrand <david@redhat.com>
---
mm/ksm.c | 158 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 157 insertions(+), 1 deletion(-)
diff --git a/mm/ksm.c b/mm/ksm.c
index 16532fa85a467..59911700695f7 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -21,6 +21,7 @@
#include <linux/sched.h>
#include <linux/sched/mm.h>
#include <linux/sched/coredump.h>
+#include <linux/sched/cputime.h>
#include <linux/rwsem.h>
#include <linux/pagemap.h>
#include <linux/rmap.h>
@@ -248,6 +249,9 @@ static struct kmem_cache *rmap_item_cache;
static struct kmem_cache *stable_node_cache;
static struct kmem_cache *mm_slot_cache;
+/* Default number of pages to scan per batch */
+#define DEFAULT_PAGES_TO_SCAN 100
+
/* The number of pages scanned */
static unsigned long ksm_pages_scanned;
@@ -276,7 +280,7 @@ static unsigned int ksm_stable_node_chains_prune_millisecs = 2000;
static int ksm_max_page_sharing = 256;
/* Number of pages ksmd should scan in one batch */
-static unsigned int ksm_thread_pages_to_scan = 100;
+static unsigned int ksm_thread_pages_to_scan = DEFAULT_PAGES_TO_SCAN;
/* Milliseconds ksmd should sleep between batches */
static unsigned int ksm_thread_sleep_millisecs = 20;
@@ -297,6 +301,152 @@ unsigned long ksm_zero_pages;
/* The number of pages that have been skipped due to "smart scanning" */
static unsigned long ksm_pages_skipped;
+/* Don't scan more than max pages per batch. */
+static unsigned long ksm_advisor_max_pages_to_scan = 30000;
+
+/* Min CPU for scanning pages per scan */
+#define KSM_ADVISOR_MIN_CPU 10
+
+/* Max CPU for scanning pages per scan */
+static unsigned int ksm_advisor_max_cpu = 70;
+
+/* Target scan time in seconds to analyze all KSM candidate pages. */
+static unsigned long ksm_advisor_target_scan_time = 200;
+
+/* Exponentially weighted moving average. */
+#define EWMA_WEIGHT 30
+
+/**
+ * struct advisor_ctx - metadata for KSM advisor
+ * @start_scan: start time of the current scan
+ * @scan_time: scan time of previous scan
+ * @change: change in percent to pages_to_scan parameter
+ * @cpu_time: cpu time consumed by the ksmd thread in the previous scan
+ */
+struct advisor_ctx {
+ ktime_t start_scan;
+ unsigned long scan_time;
+ unsigned long change;
+ unsigned long long cpu_time;
+};
+static struct advisor_ctx advisor_ctx;
+
+/* Define different advisor's */
+enum ksm_advisor_type {
+ KSM_ADVISOR_NONE,
+ KSM_ADVISOR_SCAN_TIME,
+};
+static enum ksm_advisor_type ksm_advisor;
+
+static inline void advisor_start_scan(void)
+{
+ if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
+ advisor_ctx.start_scan = ktime_get();
+}
+
+/*
+ * Use previous scan time if available, otherwise use current scan time as an
+ * approximation for the previous scan time.
+ */
+static inline unsigned long prev_scan_time(struct advisor_ctx *ctx,
+ unsigned long scan_time)
+{
+ return ctx->scan_time ? ctx->scan_time : scan_time;
+}
+
+/* Calculate exponential weighted moving average */
+static unsigned long ewma(unsigned long prev, unsigned long curr)
+{
+ return ((100 - EWMA_WEIGHT) * prev + EWMA_WEIGHT * curr) / 100;
+}
+
+/*
+ * The scan time advisor is based on the current scan rate and the target
+ * scan rate.
+ *
+ * new_pages_to_scan = pages_to_scan * (scan_time / target_scan_time)
+ *
+ * To avoid perturbations it calculates a change factor of previous changes.
+ * A new change factor is calculated for each iteration and it uses an
+ * exponentially weighted moving average. The new pages_to_scan value is
+ * multiplied with that change factor:
+ *
+ * new_pages_to_scan *= change facor
+ *
+ * The new_pages_to_scan value is limited by the cpu min and max values. It
+ * calculates the cpu percent for the last scan and calculates the new
+ * estimated cpu percent cost for the next scan. That value is capped by the
+ * cpu min and max setting.
+ *
+ * In addition the new pages_to_scan value is capped by the max and min
+ * limits.
+ */
+static void scan_time_advisor(void)
+{
+ unsigned int cpu_percent;
+ unsigned long cpu_time;
+ unsigned long cpu_time_diff;
+ unsigned long cpu_time_diff_ms;
+ unsigned long pages;
+ unsigned long per_page_cost;
+ unsigned long factor;
+ unsigned long change;
+ unsigned long last_scan_time;
+ unsigned long scan_time;
+
+ /* Convert scan time to seconds */
+ scan_time = div_s64(ktime_ms_delta(ktime_get(), advisor_ctx.start_scan),
+ MSEC_PER_SEC);
+ scan_time = scan_time ? scan_time : 1;
+
+ /* Calculate CPU consumption of ksmd background thread */
+ cpu_time = task_sched_runtime(current);
+ cpu_time_diff = cpu_time - advisor_ctx.cpu_time;
+ cpu_time_diff_ms = cpu_time_diff / 1000 / 1000;
+
+ cpu_percent = (cpu_time_diff_ms * 100) / (scan_time * 1000);
+ cpu_percent = cpu_percent ? cpu_percent : 1;
+ last_scan_time = prev_scan_time(&advisor_ctx, scan_time);
+
+ /* Calculate scan time as percentage of target scan time */
+ factor = ksm_advisor_target_scan_time * 100 / scan_time;
+ factor = factor ? factor : 1;
+
+ /*
+ * Calculate scan time as percentage of last scan time and use
+ * exponentially weighted average to smooth it
+ */
+ change = scan_time * 100 / last_scan_time;
+ change = change ? change : 1;
+ change = ewma(advisor_ctx.change, change);
+
+ /* Calculate new scan rate based on target scan rate. */
+ pages = ksm_thread_pages_to_scan * 100 / factor;
+ /* Update pages_to_scan by weighted change percentage. */
+ pages = pages * change / 100;
+
+ /* Cap new pages_to_scan value */
+ per_page_cost = ksm_thread_pages_to_scan / cpu_percent;
+ per_page_cost = per_page_cost ? per_page_cost : 1;
+
+ pages = min(pages, per_page_cost * ksm_advisor_max_cpu);
+ pages = max(pages, per_page_cost * KSM_ADVISOR_MIN_CPU);
+ pages = min(pages, ksm_advisor_max_pages_to_scan);
+
+ /* Update advisor context */
+ advisor_ctx.change = change;
+ advisor_ctx.scan_time = scan_time;
+ advisor_ctx.cpu_time = cpu_time;
+
+ ksm_thread_pages_to_scan = pages;
+}
+
+static void advisor_stop_scan(void)
+{
+ if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
+ scan_time_advisor();
+}
+
#ifdef CONFIG_NUMA
/* Zeroed when merging across nodes is not allowed */
static unsigned int ksm_merge_across_nodes = 1;
@@ -2401,6 +2551,7 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
mm_slot = ksm_scan.mm_slot;
if (mm_slot == &ksm_mm_head) {
+ advisor_start_scan();
trace_ksm_start_scan(ksm_scan.seqnr, ksm_rmap_items);
/*
@@ -2558,6 +2709,8 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
if (mm_slot != &ksm_mm_head)
goto next_mm;
+ advisor_stop_scan();
+
trace_ksm_stop_scan(ksm_scan.seqnr, ksm_rmap_items);
ksm_scan.seqnr++;
return NULL;
@@ -3248,6 +3401,9 @@ static ssize_t pages_to_scan_store(struct kobject *kobj,
unsigned int nr_pages;
int err;
+ if (ksm_advisor != KSM_ADVISOR_NONE)
+ return -EINVAL;
+
err = kstrtouint(buf, 10, &nr_pages);
if (err)
return -EINVAL;
--
2.39.3
^ permalink raw reply [flat|nested] 6+ messages in thread* [PATCH v5 2/4] mm/ksm: add sysfs knobs for advisor
2023-12-18 23:10 [PATCH v5 0/4] mm/ksm: Add ksm advisor Stefan Roesch
2023-12-18 23:10 ` [PATCH v5 1/4] mm/ksm: add " Stefan Roesch
@ 2023-12-18 23:10 ` Stefan Roesch
2023-12-18 23:10 ` [PATCH v5 3/4] mm/ksm: add tracepoint for ksm advisor Stefan Roesch
2023-12-18 23:10 ` [PATCH v5 4/4] mm/ksm: document ksm advisor and its sysfs knobs Stefan Roesch
3 siblings, 0 replies; 6+ messages in thread
From: Stefan Roesch @ 2023-12-18 23:10 UTC (permalink / raw)
To: kernel-team; +Cc: shr, akpm, david, hannes, riel, linux-kernel, linux-mm
This adds four new knobs for the KSM advisor to influence its behaviour.
The knobs are:
- advisor_mode:
none: no advisor (default)
scan-time: scan time advisor
- advisor_max_cpu: 70 (default, cpu usage percent)
- advisor_min_pages_to_scan: 500 (default)
- advisor_max_pages_to_scan: 30000 (default)
- advisor_target_scan_time: 200 (default in seconds)
The new values will take effect on the next scan round.
Signed-off-by: Stefan Roesch <shr@devkernel.io>
Acked-by: David Hildenbrand <david@redhat.com>
---
mm/ksm.c | 148 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 148 insertions(+)
diff --git a/mm/ksm.c b/mm/ksm.c
index 59911700695f7..10e2696907250 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -338,6 +338,25 @@ enum ksm_advisor_type {
};
static enum ksm_advisor_type ksm_advisor;
+#ifdef CONFIG_SYSFS
+/*
+ * Only called through the sysfs control interface:
+ */
+
+/* At least scan this many pages per batch. */
+static unsigned long ksm_advisor_min_pages_to_scan = 500;
+
+static void set_advisor_defaults(void)
+{
+ if (ksm_advisor == KSM_ADVISOR_NONE) {
+ ksm_thread_pages_to_scan = DEFAULT_PAGES_TO_SCAN;
+ } else if (ksm_advisor == KSM_ADVISOR_SCAN_TIME) {
+ advisor_ctx = (const struct advisor_ctx){ 0 };
+ ksm_thread_pages_to_scan = ksm_advisor_min_pages_to_scan;
+ }
+}
+#endif /* CONFIG_SYSFS */
+
static inline void advisor_start_scan(void)
{
if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
@@ -3723,6 +3742,130 @@ static ssize_t smart_scan_store(struct kobject *kobj,
}
KSM_ATTR(smart_scan);
+static ssize_t advisor_mode_show(struct kobject *kobj,
+ struct kobj_attribute *attr, char *buf)
+{
+ const char *output;
+
+ if (ksm_advisor == KSM_ADVISOR_NONE)
+ output = "[none] scan-time";
+ else if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
+ output = "none [scan-time]";
+
+ return sysfs_emit(buf, "%s\n", output);
+}
+
+static ssize_t advisor_mode_store(struct kobject *kobj,
+ struct kobj_attribute *attr, const char *buf,
+ size_t count)
+{
+ enum ksm_advisor_type curr_advisor = ksm_advisor;
+
+ if (sysfs_streq("scan-time", buf))
+ ksm_advisor = KSM_ADVISOR_SCAN_TIME;
+ else if (sysfs_streq("none", buf))
+ ksm_advisor = KSM_ADVISOR_NONE;
+ else
+ return -EINVAL;
+
+ /* Set advisor default values */
+ if (curr_advisor != ksm_advisor)
+ set_advisor_defaults();
+
+ return count;
+}
+KSM_ATTR(advisor_mode);
+
+static ssize_t advisor_max_cpu_show(struct kobject *kobj,
+ struct kobj_attribute *attr, char *buf)
+{
+ return sysfs_emit(buf, "%u\n", ksm_advisor_max_cpu);
+}
+
+static ssize_t advisor_max_cpu_store(struct kobject *kobj,
+ struct kobj_attribute *attr,
+ const char *buf, size_t count)
+{
+ int err;
+ unsigned long value;
+
+ err = kstrtoul(buf, 10, &value);
+ if (err)
+ return -EINVAL;
+
+ ksm_advisor_max_cpu = value;
+ return count;
+}
+KSM_ATTR(advisor_max_cpu);
+
+static ssize_t advisor_min_pages_to_scan_show(struct kobject *kobj,
+ struct kobj_attribute *attr, char *buf)
+{
+ return sysfs_emit(buf, "%lu\n", ksm_advisor_min_pages_to_scan);
+}
+
+static ssize_t advisor_min_pages_to_scan_store(struct kobject *kobj,
+ struct kobj_attribute *attr,
+ const char *buf, size_t count)
+{
+ int err;
+ unsigned long value;
+
+ err = kstrtoul(buf, 10, &value);
+ if (err)
+ return -EINVAL;
+
+ ksm_advisor_min_pages_to_scan = value;
+ return count;
+}
+KSM_ATTR(advisor_min_pages_to_scan);
+
+static ssize_t advisor_max_pages_to_scan_show(struct kobject *kobj,
+ struct kobj_attribute *attr, char *buf)
+{
+ return sysfs_emit(buf, "%lu\n", ksm_advisor_max_pages_to_scan);
+}
+
+static ssize_t advisor_max_pages_to_scan_store(struct kobject *kobj,
+ struct kobj_attribute *attr,
+ const char *buf, size_t count)
+{
+ int err;
+ unsigned long value;
+
+ err = kstrtoul(buf, 10, &value);
+ if (err)
+ return -EINVAL;
+
+ ksm_advisor_max_pages_to_scan = value;
+ return count;
+}
+KSM_ATTR(advisor_max_pages_to_scan);
+
+static ssize_t advisor_target_scan_time_show(struct kobject *kobj,
+ struct kobj_attribute *attr, char *buf)
+{
+ return sysfs_emit(buf, "%lu\n", ksm_advisor_target_scan_time);
+}
+
+static ssize_t advisor_target_scan_time_store(struct kobject *kobj,
+ struct kobj_attribute *attr,
+ const char *buf, size_t count)
+{
+ int err;
+ unsigned long value;
+
+ err = kstrtoul(buf, 10, &value);
+ if (err)
+ return -EINVAL;
+ if (value < 1)
+ return -EINVAL;
+
+ ksm_advisor_target_scan_time = value;
+ return count;
+}
+KSM_ATTR(advisor_target_scan_time);
+
static struct attribute *ksm_attrs[] = {
&sleep_millisecs_attr.attr,
&pages_to_scan_attr.attr,
@@ -3745,6 +3888,11 @@ static struct attribute *ksm_attrs[] = {
&use_zero_pages_attr.attr,
&general_profit_attr.attr,
&smart_scan_attr.attr,
+ &advisor_mode_attr.attr,
+ &advisor_max_cpu_attr.attr,
+ &advisor_min_pages_to_scan_attr.attr,
+ &advisor_max_pages_to_scan_attr.attr,
+ &advisor_target_scan_time_attr.attr,
NULL,
};
--
2.39.3
^ permalink raw reply [flat|nested] 6+ messages in thread* [PATCH v5 3/4] mm/ksm: add tracepoint for ksm advisor
2023-12-18 23:10 [PATCH v5 0/4] mm/ksm: Add ksm advisor Stefan Roesch
2023-12-18 23:10 ` [PATCH v5 1/4] mm/ksm: add " Stefan Roesch
2023-12-18 23:10 ` [PATCH v5 2/4] mm/ksm: add sysfs knobs for advisor Stefan Roesch
@ 2023-12-18 23:10 ` Stefan Roesch
2023-12-18 23:10 ` [PATCH v5 4/4] mm/ksm: document ksm advisor and its sysfs knobs Stefan Roesch
3 siblings, 0 replies; 6+ messages in thread
From: Stefan Roesch @ 2023-12-18 23:10 UTC (permalink / raw)
To: kernel-team; +Cc: shr, akpm, david, hannes, riel, linux-kernel, linux-mm
This adds a new tracepoint for the ksm advisor. It reports the last scan
time, the new setting of the pages_to_scan parameter and the average cpu
percent usage of the ksmd background thread for the last scan.
Signed-off-by: Stefan Roesch <shr@devkernel.io>
Acked-by: David Hildenbrand <david@redhat.com>
---
include/trace/events/ksm.h | 33 +++++++++++++++++++++++++++++++++
mm/ksm.c | 1 +
2 files changed, 34 insertions(+)
diff --git a/include/trace/events/ksm.h b/include/trace/events/ksm.h
index b5ac35c1d0e88..e728647b5d268 100644
--- a/include/trace/events/ksm.h
+++ b/include/trace/events/ksm.h
@@ -245,6 +245,39 @@ TRACE_EVENT(ksm_remove_rmap_item,
__entry->pfn, __entry->rmap_item, __entry->mm)
);
+/**
+ * ksm_advisor - called after the advisor has run
+ *
+ * @scan_time: scan time in seconds
+ * @pages_to_scan: new pages_to_scan value
+ * @cpu_percent: cpu usage in percent
+ *
+ * Allows to trace the ksm advisor.
+ */
+TRACE_EVENT(ksm_advisor,
+
+ TP_PROTO(s64 scan_time, unsigned long pages_to_scan,
+ unsigned int cpu_percent),
+
+ TP_ARGS(scan_time, pages_to_scan, cpu_percent),
+
+ TP_STRUCT__entry(
+ __field(s64, scan_time)
+ __field(unsigned long, pages_to_scan)
+ __field(unsigned int, cpu_percent)
+ ),
+
+ TP_fast_assign(
+ __entry->scan_time = scan_time;
+ __entry->pages_to_scan = pages_to_scan;
+ __entry->cpu_percent = cpu_percent;
+ ),
+
+ TP_printk("ksm scan time %lld pages_to_scan %lu cpu percent %u",
+ __entry->scan_time, __entry->pages_to_scan,
+ __entry->cpu_percent)
+);
+
#endif /* _TRACE_KSM_H */
/* This part must be outside protection */
diff --git a/mm/ksm.c b/mm/ksm.c
index 10e2696907250..82688fc6f119f 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -458,6 +458,7 @@ static void scan_time_advisor(void)
advisor_ctx.cpu_time = cpu_time;
ksm_thread_pages_to_scan = pages;
+ trace_ksm_advisor(scan_time, pages, cpu_percent);
}
static void advisor_stop_scan(void)
--
2.39.3
^ permalink raw reply [flat|nested] 6+ messages in thread* [PATCH v5 4/4] mm/ksm: document ksm advisor and its sysfs knobs
2023-12-18 23:10 [PATCH v5 0/4] mm/ksm: Add ksm advisor Stefan Roesch
` (2 preceding siblings ...)
2023-12-18 23:10 ` [PATCH v5 3/4] mm/ksm: add tracepoint for ksm advisor Stefan Roesch
@ 2023-12-18 23:10 ` Stefan Roesch
2023-12-19 23:53 ` Randy Dunlap
3 siblings, 1 reply; 6+ messages in thread
From: Stefan Roesch @ 2023-12-18 23:10 UTC (permalink / raw)
To: kernel-team; +Cc: shr, akpm, david, hannes, riel, linux-kernel, linux-mm
This documents the KSM advisor and its new knobs in /sys/fs/kernel/mm.
Signed-off-by: Stefan Roesch <shr@devkernel.io>
Acked-by: David Hildenbrand <david@redhat.com>
---
Documentation/admin-guide/mm/ksm.rst | 55 ++++++++++++++++++++++++++++
1 file changed, 55 insertions(+)
diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
index e59231ac6bb71..a639cac124777 100644
--- a/Documentation/admin-guide/mm/ksm.rst
+++ b/Documentation/admin-guide/mm/ksm.rst
@@ -80,6 +80,9 @@ pages_to_scan
how many pages to scan before ksmd goes to sleep
e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan``.
+ The pages_to_scan value cannot be changed if ``advisor_mode`` has
+ been set to scan-time.
+
Default: 100 (chosen for demonstration purposes)
sleep_millisecs
@@ -164,6 +167,29 @@ smart_scan
optimization is enabled. The ``pages_skipped`` metric shows how
effective the setting is.
+advisor_mode
+ The ``advisor_mode`` selects the current advisor. Two modes are
+ supported: none and scan-time. The default is none. By setting
+ ``advisor_mode`` to scan-time, the scan time advisor is enabled.
+ The section about ``advisor`` explains in detail how the scan time
+ advisor works.
+
+adivsor_max_cpu
+ specifies the upper limit of the cpu percent usage of the ksmd
+ background thread. The default is 70.
+
+advisor_target_scan_time
+ specifies the target scan time in seconds to scan all the candidate
+ pages. The default value is 200 seconds.
+
+advisor_min_pages_to_scan
+ specifies the lower limit of the ``pages_to_scan`` parameter of the
+ scan time advisor. The default is 500.
+
+adivsor_max_pages_to_scan
+ specifies the upper limit of the ``pages_to_scan`` parameter of the
+ scan time advisor. The default is 30000.
+
The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
general_profit
@@ -263,6 +289,35 @@ ksm_swpin_copy
note that KSM page might be copied when swapping in because do_swap_page()
cannot do all the locking needed to reconstitute a cross-anon_vma KSM page.
+Advisor
+=======
+
+The number of candidate pages for KSM is dynamic. It can be often observed
+that during the startup of an application more candidate pages need to be
+processed. Without an advisor the ``pages_to_scan`` parameter needs to be
+sized for the maximum number of candidate pages. The scan time advisor can
+changes the ``pages_to_scan`` parameter based on demand.
+
+The advisor can be enabled, so KSM can automatically adapt to changes in the
+number of candidate pages to scan. Two advisors are implemented: none and
+scan-time. With none, no advisor is enabled. The default is none.
+
+The scan time advisor changes the ``pages_to_scan`` parameter based on the
+observed scan times. The possible values for the ``pages_to_scan`` parameter is
+limited by the ``advisor_max_cpu`` parameter. In addition there is also the
+``advisor_target_scan_time`` parameter. This parameter sets the target time to
+scan all the KSM candidate pages. The parameter ``advisor_target_scan_time``
+decides how aggressive the scan time advisor scans candidate pages. Lower
+values make the scan time advisor to scan more aggresively. This is the most
+important parameter for the configuration of the scan time advisor.
+
+The initial value and the maximum value can be changed with
+``advisor_min_pages_to_scan`` and ``advisor_max_pages_to_scan``. The default
+values are sufficient for most workloads and use cases.
+
+The ``pages_to_scan`` parameter is re-calculated after a scan has been completed.
+
+
--
Izik Eidus,
Hugh Dickins, 17 Nov 2009
--
2.39.3
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH v5 4/4] mm/ksm: document ksm advisor and its sysfs knobs
2023-12-18 23:10 ` [PATCH v5 4/4] mm/ksm: document ksm advisor and its sysfs knobs Stefan Roesch
@ 2023-12-19 23:53 ` Randy Dunlap
0 siblings, 0 replies; 6+ messages in thread
From: Randy Dunlap @ 2023-12-19 23:53 UTC (permalink / raw)
To: Stefan Roesch, kernel-team
Cc: akpm, david, hannes, riel, linux-kernel, linux-mm
Hi,
On 12/18/23 15:10, Stefan Roesch wrote:
> This documents the KSM advisor and its new knobs in /sys/fs/kernel/mm.
>
> Signed-off-by: Stefan Roesch <shr@devkernel.io>
> Acked-by: David Hildenbrand <david@redhat.com>
> ---
> Documentation/admin-guide/mm/ksm.rst | 55 ++++++++++++++++++++++++++++
> 1 file changed, 55 insertions(+)
>
> diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
> index e59231ac6bb71..a639cac124777 100644
> --- a/Documentation/admin-guide/mm/ksm.rst
> +++ b/Documentation/admin-guide/mm/ksm.rst
> @@ -80,6 +80,9 @@ pages_to_scan
> how many pages to scan before ksmd goes to sleep
> e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan``.
>
> + The pages_to_scan value cannot be changed if ``advisor_mode`` has
> + been set to scan-time.
> +
> Default: 100 (chosen for demonstration purposes)
>
> sleep_millisecs
> @@ -164,6 +167,29 @@ smart_scan
> optimization is enabled. The ``pages_skipped`` metric shows how
> effective the setting is.
>
> +advisor_mode
> + The ``advisor_mode`` selects the current advisor. Two modes are
> + supported: none and scan-time. The default is none. By setting
> + ``advisor_mode`` to scan-time, the scan time advisor is enabled.
> + The section about ``advisor`` explains in detail how the scan time
scan-time
> + advisor works.
> +
> +adivsor_max_cpu
> + specifies the upper limit of the cpu percent usage of the ksmd
> + background thread. The default is 70.
> +
> +advisor_target_scan_time
> + specifies the target scan time in seconds to scan all the candidate
> + pages. The default value is 200 seconds.
> +
> +advisor_min_pages_to_scan
> + specifies the lower limit of the ``pages_to_scan`` parameter of the
> + scan time advisor. The default is 500.
> +
> +adivsor_max_pages_to_scan
> + specifies the upper limit of the ``pages_to_scan`` parameter of the
> + scan time advisor. The default is 30000.
scan-time
> +
> The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
>
> general_profit
> @@ -263,6 +289,35 @@ ksm_swpin_copy
> note that KSM page might be copied when swapping in because do_swap_page()
> cannot do all the locking needed to reconstitute a cross-anon_vma KSM page.
>
> +Advisor
> +=======
> +
> +The number of candidate pages for KSM is dynamic. It can be often observed
> +that during the startup of an application more candidate pages need to be
> +processed. Without an advisor the ``pages_to_scan`` parameter needs to be
> +sized for the maximum number of candidate pages. The scan time advisor can
> +changes the ``pages_to_scan`` parameter based on demand.
change
> +
> +The advisor can be enabled, so KSM can automatically adapt to changes in the
> +number of candidate pages to scan. Two advisors are implemented: none and
> +scan-time. With none, no advisor is enabled. The default is none.
> +
> +The scan time advisor changes the ``pages_to_scan`` parameter based on the
scan-time
> +observed scan times. The possible values for the ``pages_to_scan`` parameter is
> +limited by the ``advisor_max_cpu`` parameter. In addition there is also the
> +``advisor_target_scan_time`` parameter. This parameter sets the target time to
> +scan all the KSM candidate pages. The parameter ``advisor_target_scan_time``
> +decides how aggressive the scan time advisor scans candidate pages. Lower
> +values make the scan time advisor to scan more aggresively. This is the most
I would drop to. aggressively
> +important parameter for the configuration of the scan time advisor.
scan-time
> +
> +The initial value and the maximum value can be changed with
> +``advisor_min_pages_to_scan`` and ``advisor_max_pages_to_scan``. The default
> +values are sufficient for most workloads and use cases.
> +
> +The ``pages_to_scan`` parameter is re-calculated after a scan has been completed.
recalculated
> +
> +
> --
> Izik Eidus,
> Hugh Dickins, 17 Nov 2009
thanks.
--
#Randy
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