[Qemu-devel] [PATCH v1 08/18] block/pcache: updating statistics for overlapping requests

[Pavel Butsykin]

When updating the statistics sometimes i/O requests can overlap each other,
in this case the requests are not stored in the statistics. It's not very good,
especially when the requests have a small range of intersection.

We can cut the requests in the intersection and add the pieces of requests in
the statistics. Maybe not significantly, but it can make the statistics more
accurate. Here, update_req_stats() adds the ability to save overlapping
requests.

Signed-off-by: Pavel Butsykin <address@hidden>
---
 block/pcache.c | 36 +++++++++++++++++++++++++++++++++++-
 1 file changed, 35 insertions(+), 1 deletion(-)

diff --git a/block/pcache.c b/block/pcache.c
index bfc7e97..dd598f3 100644
--- a/block/pcache.c
+++ b/block/pcache.c
@@ -64,6 +64,40 @@ static void pcache_aio_cb(void *opaque, int ret)
     qemu_coroutine_enter(acb->co);
 }
 
+static void update_req_stats(RBCache *rbcache, uint64_t offset, uint64_t bytes)
+{
+    do {
+        RBCacheNode *node = rbcache_search_and_insert(rbcache, offset, bytes);
+        /* The node was successfully added or already exists */
+        if (node->offset <= offset &&
+            node->offset + node->bytes >= offset + bytes)
+        {
+            break;
+        }
+
+        /* Request covers the whole node */
+        if (offset <= node->offset &&
+            offset + bytes >= node->offset + node->bytes)
+        {
+            rbcache_remove(rbcache, node);
+            continue;
+        }
+
+        if (offset < node->offset) {
+            RBCacheNode *new_node =
+                rbcache_node_alloc(rbcache, offset, node->offset - offset);
+            if (new_node != rbcache_insert(rbcache, new_node)) {
+                /* The presence of the node in this range is impossible */
+                abort();
+            }
+            break;
+        }
+
+        bytes = (offset + bytes) - (node->offset + node->bytes);
+        offset = node->offset + node->bytes;
+    } while (true);
+}
+
 static coroutine_fn int pcache_co_preadv(BlockDriverState *bs, uint64_t offset,
                                          uint64_t bytes, QEMUIOVector *qiov,
                                          int flags)
@@ -74,7 +108,7 @@ static coroutine_fn int pcache_co_preadv(BlockDriverState 
*bs, uint64_t offset,
     };
 
     if (s->max_aio_size >= bytes) {
-        rbcache_search_and_insert(s->req_stats, offset, bytes);
+        update_req_stats(s->req_stats, offset, bytes);
     }
 
     bdrv_aio_preadv(bs->file, offset, qiov, bytes, pcache_aio_cb, &acb);
-- 
2.10.1