WT-2391: De-prioritize eviction from indexes

With commit 7f84e3b it's unlikely for a page to NOT have a read
generation, it only happens if a reader configured with WT_READ_NO_GEN
set were to read a page into cache, and that won't happen because those
readers set either WT_READ_CACHE or WT_READ_WONT_NEED.

The eviction code still has special code to handle pages without a read
generation set, and it maintains a "oldest page generation" value to
support that. However, the oldest page generation value isn't really
right -- it's simply that page generation value from the first slot in
the queue, which can be special (for example, if the page generation was
set to WT_READGEN_OLDEST because the page was empty).

Add a test in the page-read code to set the page generation if it's
never been set, regardless of WT_READ_NO_GEN.

Remove the eviction code to maintain an "oldest page generation".

Leave the eviction code to handle WT_READGEN_NOTSET (it can still see
that value if there's a race), but change it to set the page read
generation to the "standard" value instead of trying anything fancy.
That ensure that if there's a bug (we read the page but for some reason
never set the page generation), the page won't stay in that state
forever.

4 files changed, 16 insertions, 27 deletions

diff --git a/src/btree/bt_read.c b/src/btree/bt_read.c
index ac9faef4ff2..fd84ad8c7c8 100644
--- a/src/btree/bt_read.c
+++ b/src/btree/bt_read.c
@@ -575,18 +575,23 @@ __wt_page_in_func(WT_SESSION_IMPL *session, WT_REF *ref, uint32_t flags
 			}
 
 			/*
-			 * If we read the page and we are configured to not
-			 * trash the cache, set the oldest read generation so
-			 * the page is forcibly evicted as soon as possible.
+			 * If we read the page and are configured to not trash
+			 * the cache, and no other thread has already used the
+			 * page, set the oldest read generation so the page is
+			 * forcibly evicted as soon as possible.
 			 *
-			 * Otherwise, update the page's read generation.
+			 * Otherwise, if we read the page, or, if configured to
+			 * update the page's read generation and the page isn't
+			 * already flagged for forced eviction, update the page
+			 * read generation.
 			 */
 			page = ref->page;
 			if (oldgen && page->read_gen == WT_READGEN_NOTSET)
 				__wt_page_evict_soon(page);
-			else if (!LF_ISSET(WT_READ_NO_GEN) &&
+			else if (page->read_gen == WT_READGEN_NOTSET ||
+			    (!LF_ISSET(WT_READ_NO_GEN) &&
 			    page->read_gen != WT_READGEN_OLDEST &&
-			    page->read_gen < __wt_cache_read_gen(session))
+			    page->read_gen < __wt_cache_read_gen(session)))
 				page->read_gen =
 				    __wt_cache_read_gen_bump(session);
 skip_evict:
diff --git a/src/evict/evict_lru.c b/src/evict/evict_lru.c
index 884c08a02df..581637f1396 100644
--- a/src/evict/evict_lru.c
+++ b/src/evict/evict_lru.c
@@ -933,9 +933,6 @@ __evict_lru_walk(WT_SESSION_IMPL *session)
 
 	WT_ASSERT(session, cache->evict_queue[0].ref != NULL);
 
-	/* Track the oldest read generation we have in the queue. */
-	cache->read_gen_oldest = cache->evict_queue[0].ref->page->read_gen;
-
 	if (FLD_ISSET(cache->state,
 	    WT_EVICT_PASS_AGGRESSIVE | WT_EVICT_PASS_WOULD_BLOCK))
 		/*
@@ -1312,11 +1309,13 @@ __evict_walk_file(WT_SESSION_IMPL *session, u_int *slotp)
 			continue;
 
 		/*
-		 * If this page has never been considered for eviction, set its
-		 * read generation to somewhere in the middle of the LRU list.
+		 * It's possible (but unlikely) to visit a page without a read
+		 * generation, if we race with the read instantiating the page.
+		 * Be safe and set the page's read generation here to ensure a
+		 * bug doesn't somehow leave a page without a read generation.
 		 */
 		if (page->read_gen == WT_READGEN_NOTSET)
-			page->read_gen = __wt_cache_read_gen_new(session);
+			page->read_gen = __wt_cache_read_gen_bump(session);
 
 fast:		/* If the page can't be evicted, give up. */
 		if (!__wt_page_can_evict(session, ref, NULL))
diff --git a/src/include/cache.h b/src/include/cache.h
index a3961d6043e..fb221fd9af5 100644
--- a/src/include/cache.h
+++ b/src/include/cache.h
@@ -76,8 +76,6 @@ struct __wt_cache {
 	 * Read information.
 	 */
 	uint64_t   read_gen;		/* Page read generation (LRU) */
-	uint64_t   read_gen_oldest;	/* The oldest read generation that
-					   eviction knows about */
 
 	/*
 	 * Eviction thread information.
diff --git a/src/include/cache.i b/src/include/cache.i
index ee13eee84c5..46809598d5c 100644
--- a/src/include/cache.i
+++ b/src/include/cache.i
@@ -46,19 +46,6 @@ __wt_cache_read_gen_bump(WT_SESSION_IMPL *session)
 }
 
 /*
- * __wt_cache_read_gen_new --
- *      Get the read generation for a new page in memory.
- */
-static inline uint64_t
-__wt_cache_read_gen_new(WT_SESSION_IMPL *session)
-{
-	WT_CACHE *cache;
-
-	cache = S2C(session)->cache;
-	return (__wt_cache_read_gen(session) + cache->read_gen_oldest) / 2;
-}
-
-/*
  * __wt_cache_pages_inuse --
  *	Return the number of pages in use.
  */