On 13.12.23 19:27, Stefan Roesch wrote:
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@xxxxxxxxxxxx>
---
mm/ksm.c | 161 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 160 insertions(+), 1 deletion(-)
diff --git a/mm/ksm.c b/mm/ksm.c
index 7efcc68ccc6ea..4f7b71a1f3112 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 +301,155 @@ 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 = 30000;
+
+/* At least scan this many pages per batch. */
+static unsigned long ksm_advisor_min_pages = 500;
+
+/* Min CPU for scanning pages per scan */
+static unsigned int ksm_advisor_min_cpu = 10;
That will never be modified, right? Either mark it const or just turn it
into a define.
[...]
+/*
+ * 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);
I'd simply inline prev_scan_time() here and get rid of it. Whatever you
think is best.
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
--
Cheers,
David / dhildenb
