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 | 67 +++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 65 insertions(+), 2 deletions(-)
diff --git a/migration/ram.c b/migration/ram.c
index bcdccdaef7..d5b50b7804 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -322,6 +322,10 @@ 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 */
@@ -1506,6 +1510,8 @@ static RAMBlock *poll_fault_page(RAMState *rs, ram_addr_t
*offset)
return NULL;
}
+ rs->last_fault_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
+
*offset = (ram_addr_t) (page_address - (hwaddr) bs->host);
return bs;
}
@@ -1882,6 +1888,55 @@ static void ram_save_host_page_post(RAMState *rs,
PageSearchStatus *pss,
}
}
+#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)
+{
+ if (!migrate_background_snapshot()) {
+ return;
+ }
+
+#ifdef CONFIG_LINUX
+ int64_t last_fault_latency_ns = 0;
+
+ /* 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--;
+ }
+#else
+ /* Should never happen */
+ qemu_file_set_error(rs->f, -ENOSYS);
+#endif /* CONFIG_LINUX */
+}
+
/**
* ram_find_and_save_block: finds a dirty page and sends it to f
*
@@ -1931,6 +1986,9 @@ static int ram_find_and_save_block(RAMState *rs, bool
last_stage)
ram_save_host_page_pre(rs, &pss, &opaque);
pages = ram_save_host_page(rs, &pss, last_stage);
ram_save_host_page_post(rs, &pss, opaque, &pages);
+
+ /* Linear scan rate limiting */
+ limit_scan_rate(rs);
}
} while (!pages && again);
@@ -2043,11 +2101,14 @@ 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_bulk_stage = true;
rs->fpo_enabled = false;
}
-#define MAX_WAIT 50 /* ms, half buffered_file limit */
+#define MAX_WAIT 50 /* ms, half buffered_file limit */
+#define BG_MAX_WAIT 1000 /* 1000 ms, need bigger limit for background
snapshot */
/*
* 'expected' is the value you expect the bitmap mostly to be full
@@ -2723,7 +2784,9 @@ 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 = migrate_background_snapshot() ?
+ BG_MAX_WAIT : MAX_WAIT;
+ if (t1 > max_wait) {
trace_ram_save_iterate_big_wait(t1, i);
break;
}
--
2.25.1
