-
Type:
Bug
-
Resolution: Done
-
Priority:
Major - P3
-
None
-
Affects Version/s: None
-
Component/s: None
Large numbers of updates to a single key in the presence of a snapshot (or any old reader), can significantly slow WiredTiger. Here's the test:
def test_las_update(self):
# Create a small table.
uri = "table:test_las_update"
nrows = 100
ds = SimpleDataSet(self, uri, nrows, key_format="S")
ds.populate()
# Take a snapshot.
self.session.snapshot("name=xxx")
# Update a record a large number of times, we'll hang if the lookaside
# table isn't doing its thing.
c = self.session.open_cursor(uri)
bigvalue = "a" * 100
for i in range(1, 50000):
if i % 1000 == 0:
self.tty(str(i))
c.set_key(ds.key(nrows + 1))
c.set_value(bigvalue)
self.assertEquals(c.insert(), 0)
The large number of updates on a single page eventually triggers forced eviction, but because there's only a single "insert" on the page, we don't trigger an in-memory split, and we fall through to normal reconciliation, which returns EBUSY plus a lookaside table recommendation.
The lookaside table isn't currently called because the cache isn't "stuck". Adding a check on forced eviction fixes that problem, and we can evict the page.
diff --git a/src/btree/bt_read.c b/src/btree/bt_read.c
index 72a69e859..358f433ad 100644
--- a/src/btree/bt_read.c
+++ b/src/btree/bt_read.c
@@ -292,11 +292,11 @@ err: WT_TRET(__wt_las_cursor_close(session, &cursor, session_flags));
}
/*
- * __evict_force_check --
+ * __wt_evict_force_check --
* Check if a page matches the criteria for forced eviction.
*/
-static bool
-__evict_force_check(WT_SESSION_IMPL *session, WT_REF *ref)
+bool
+__wt_evict_force_check(WT_SESSION_IMPL *session, WT_REF *ref)
{
WT_BTREE *btree;
WT_PAGE *page;
@@ -595,7 +595,7 @@ __wt_page_in_func(WT_SESSION_IMPL *session, WT_REF *ref, uint32_t flags
* Forcibly evict pages that are too big.
*/
if (force_attempts < 10 &&
- __evict_force_check(session, ref)) {
+ __wt_evict_force_check(session, ref)) {
++force_attempts;
ret = __wt_page_release_evict(session, ref);
/* If forced eviction fails, stall. */
diff --git a/src/evict/evict_page.c b/src/evict/evict_page.c
index edcd108e7..3a6ba0591 100644
--- a/src/evict/evict_page.c
+++ b/src/evict/evict_page.c
@@ -547,7 +547,9 @@ __evict_review(
* lookaside table, allowing the eviction of pages we'd otherwise have
* to retain in cache to support older readers.
*/
- if (ret == EBUSY && __wt_cache_stuck(session) && lookaside_retry) {
+ if (ret == EBUSY && lookaside_retry &&
+ (__wt_cache_stuck(session) ||
+ __wt_evict_force_check(session, ref))) {
LF_CLR(WT_EVICT_SCRUB | WT_EVICT_UPDATE_RESTORE);
LF_SET(WT_EVICT_LOOKASIDE);
ret = __wt_reconcile(session, ref, NULL, flags, NULL);
However, the next update to the page reads the page back into memory, the lookaside table is read, and the too-big update chain re-instantiated in memory, and we're right back where we started.