Since reading UFFD events and saving paged data are performed
from the same thread, write fault latencies are sensitive to
migration stream stalls. Limiting total page saving rate is a
method to reduce amount of noticiable fault resolution latencies.
Migration bandwidth limiting is achieved via noticing cases of
out-of-threshold write fault latencies and temporarily disabling
(strictly speaking, severely throttling) saving non-faulting pages.
Signed-off-by: Andrey Gruzdev <andrey.gruz...@virtuozzo.com>
---
migration/ram.c | 58 +++++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 54 insertions(+), 4 deletions(-)
diff --git a/migration/ram.c b/migration/ram.c
index 08a1d7a252..89fe106585 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -325,6 +325,10 @@ struct RAMState {
/* these variables are used for bitmap sync */
/* last time we did a full bitmap_sync */
int64_t time_last_bitmap_sync;
+ /* last time UFFD fault occured */
+ int64_t last_fault_ns;
+ /* linear scan throttling counter */
+ int throttle_skip_counter;
/* bytes transferred at start_time */
uint64_t bytes_xfer_prev;
/* number of dirty pages since start_time */
@@ -576,9 +580,6 @@ static int uffd_protect_memory(int uffd, hwaddr start,
hwaddr length, bool wp)
return 0;
}
-__attribute__ ((unused))
-static bool uffd_poll_events(int uffd, int tmo);
-
/**
* uffd_read_events: read pending UFFD events
*
@@ -2006,9 +2007,51 @@ static bool get_fault_page(RAMState *rs,
PageSearchStatus *pss)
return false;
}
+ rs->last_fault_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
return true;
}
+#define FAULT_HIGH_LATENCY_NS 5000000 /* 5 ms */
+#define SLOW_FAULT_POLL_TMO 5 /* 5 ms */
+#define SLOW_FAULT_SKIP_PAGES 200
+
+/**
+ * limit_scan_rate: limit RAM linear scan rate in case of growing write fault
+ * latencies, used in write-tracking migration implementation
+ *
+ * @rs: current RAM state
+ *
+ */
+static void limit_scan_rate(RAMState *rs)
+{
+ int64_t last_fault_latency_ns = 0;
+
+ if (!rs->ram_wt_enabled) {
+ return;
+ }
+
+ /* Check if last write fault time is available */
+ if (rs->last_fault_ns) {
+ last_fault_latency_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) -
+ rs->last_fault_ns;
+ rs->last_fault_ns = 0;
+ }
+
+ /* In case last fault time was available and we have
+ * latency value, check if it's not too high */
+ if (last_fault_latency_ns > FAULT_HIGH_LATENCY_NS) {
+ /* Reset counter after each slow write fault */
+ rs->throttle_skip_counter = SLOW_FAULT_SKIP_PAGES;
+ }
+ /* Delay thread execution till next write fault occures or timeout expires.
+ * Next SLOW_FAULT_SKIP_PAGES can be write fault pages only, not from
pages going from
+ * linear scan logic. Thus we moderate migration stream rate to reduce
latencies */
+ if (rs->throttle_skip_counter > 0) {
+ uffd_poll_events(rs->uffdio_fd, SLOW_FAULT_POLL_TMO);
+ rs->throttle_skip_counter--;
+ }
+}
+
/**
* ram_find_and_save_block: finds a dirty page and sends it to f
*
@@ -2078,6 +2121,9 @@ static int ram_find_and_save_block(RAMState *rs, bool
last_stage)
if (res < 0) {
break;
}
+
+ /* Linear scan rate limiting */
+ limit_scan_rate(rs);
}
}
} while (!pages && again);
@@ -2191,12 +2237,15 @@ static void ram_state_reset(RAMState *rs)
rs->last_sent_block = NULL;
rs->last_page = 0;
rs->last_version = ram_list.version;
+ rs->last_fault_ns = 0;
+ rs->throttle_skip_counter = 0;
rs->ram_wt_enabled = migrate_track_writes_ram();
rs->ram_bulk_stage = !rs->ram_wt_enabled;
rs->fpo_enabled = false;
}
#define MAX_WAIT 50 /* ms, half buffered_file limit */
+#define WT_MAX_WAIT 1000 /* 1000 ms, need bigger limit for 'write-tracking'
migration */
/*
* 'expected' is the value you expect the bitmap mostly to be full
@@ -2872,7 +2921,8 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
if ((i & 63) == 0) {
uint64_t t1 = (qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - t0) /
1000000;
- if (t1 > MAX_WAIT) {
+ uint64_t max_wait = rs->ram_wt_enabled ? WT_MAX_WAIT :
MAX_WAIT;
+ if (t1 > max_wait) {
trace_ram_save_iterate_big_wait(t1, i);
break;
}
--
2.25.1
