On 28.10.23 02:09, 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
- to avoid to spend too much CPU
The respective parameters are:
- ksm_advisor_target_scan_time (how many seconds a scan should take)
- ksm_advisor_min_cpu (minimum value for cpu percent usage)
- 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 three main parameters: target scan time,
cpu min time and 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
- min cpu: 15%
- max cpu: 70%
- min pages: 500
- max pages: 30000
Do we really need the min cpu load? The target scan time combined with
the max CPU load should be sufficient, no?
Internally, we might want some sane default/min start value, but
exposing that to the user is questionable.
For example, if I have exactly two possible KSM pages in the system, why
should my cpu dedicate 15% to scanning nothing after merging them? :)
[...]
+/**
+ * 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_percent: average cpu percent usage of the ksmd thread for the last 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_FIRST = KSM_ADVISOR_NONE,
Unused, better drop it. 0 is the implicit first one.
+ KSM_ADVISOR_SCAN_TIME,
+ KSM_ADVISOR_LAST = KSM_ADVISOR_SCAN_TIME
Instead of "_LAST", maybe use "_COUNT" and use that when checking for
valid values.
But: we likely want to store "strings" instead of magic numbers from
user space instead.
+};
+static enum ksm_advisor_type ksm_advisor;
+
+static void init_advisor(void)
+{
+ advisor_ctx.start_scan = 0;
+ advisor_ctx.scan_time = 0;
+ advisor_ctx.change = 0;
+ advisor_ctx.cpu_time = 0;
+}
That should likely not be required. The values are all 0.
If other values are ever required, they could be initialized right with
the variable:
static struct advisor_ctx advisor_ctx = {
.start_scan = 0,
...
};
+
+/*
+ * 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 pertubations 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
+ *
+ * In addition the new pages_to_scan value is capped by the max and min
+ * limits.
+ */
+static void scan_time_advisor(unsigned long scan_time)
+{
+ 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;
+
+ 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);
+
+ /* 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;
While that advisor is active, we should likely disallow changing
ksm_thread_pages_to_scan using other means.
+}
+
+static void run_advisor(void)
+{
+ if (ksm_advisor == KSM_ADVISOR_SCAN_TIME) {
+ s64 scan_time;
+
+ /* Convert scan time to seconds */
+ scan_time = ktime_ms_delta(ktime_get(), advisor_ctx.start_scan);
+ scan_time = div_s64(scan_time, MSEC_PER_SEC);
+ scan_time = scan_time ? scan_time : 1;
+
+ scan_time_advisor((unsigned long)scan_time);
+ }
We could have rescheduled in the meantime, right? Doesn't that mean that
our CPU load consumption might be wrong in some cases?
+}
+
#ifdef CONFIG_NUMA
/* Zeroed when merging across nodes is not allowed */
static unsigned int ksm_merge_across_nodes = 1;
@@ -2401,6 +2554,7 @@ 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_ctx.start_scan = ktime_get();
Why do that even without KSM_ADVISOR_SCAN_TIME?
You should probably have two functions:
ksm_advisor_start_scan() [this code, fenced by KSM_ADVISOR_SCAN_TIME]
ksm_advisor_stop_scan() [previous run_advisor]
trace_ksm_start_scan(ksm_scan.seqnr, ksm_rmap_items);
/*
@@ -2558,6 +2712,8 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
if (mm_slot != &ksm_mm_head)
goto next_mm;
+ run_advisor();
+
trace_ksm_stop_scan(ksm_scan.seqnr, ksm_rmap_items);
ksm_scan.seqnr++;
return NULL;
@@ -3603,6 +3759,7 @@ static int __init ksm_init(void)
zero_checksum = calc_checksum(ZERO_PAGE(0));
/* Default to false for backwards compatibility */
ksm_use_zero_pages = false;
+ init_advisor();
err = ksm_slab_init();
if (err)
--
Cheers,
David / dhildenb
