[Qemu-devel] [RFC PATCH] qht: Align sequence lock to cache line

[Pranith Kumar <bobby.prani@gmail.com>]

Using perf, I see that sequence lock is being a bottleneck since it is
being read by everyone. Giving it its own cache-line seems to help
things quite a bit.

Using qht-bench, I measured the following for:

$ ./tests/qht-bench -d 10 -n 24 -u <x>

throughput base   patch  %change
update
0          8.07   13.33  +65%
10         7.10   8.90   +25%
20         6.34   7.02	 +10%
30         5.48   6.11   +9.6%
40         4.90   5.46   +11.42%

I am not able to see any significant increases for lower thread counts though.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
---
 include/qemu/seqlock.h | 2 +-
 util/qht.c             | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/qemu/seqlock.h b/include/qemu/seqlock.h
index 8dee11d..954abe8 100644
--- a/include/qemu/seqlock.h
+++ b/include/qemu/seqlock.h
@@ -21,7 +21,7 @@ typedef struct QemuSeqLock QemuSeqLock;
 
 struct QemuSeqLock {
     unsigned sequence;
-};
+} QEMU_ALIGNED(64);
 
 static inline void seqlock_init(QemuSeqLock *sl)
 {
diff --git a/util/qht.c b/util/qht.c
index ff4d2e6..4d82609 100644
--- a/util/qht.c
+++ b/util/qht.c
@@ -101,14 +101,14 @@
  * be grabbed first.
  */
 struct qht_bucket {
-    QemuSpin lock;
     QemuSeqLock sequence;
+    QemuSpin lock;
     uint32_t hashes[QHT_BUCKET_ENTRIES];
     void *pointers[QHT_BUCKET_ENTRIES];
     struct qht_bucket *next;
 } QEMU_ALIGNED(QHT_BUCKET_ALIGN);
 
-QEMU_BUILD_BUG_ON(sizeof(struct qht_bucket) > QHT_BUCKET_ALIGN);
+QEMU_BUILD_BUG_ON(sizeof(struct qht_bucket) > 2 * QHT_BUCKET_ALIGN);
 
 /**
  * struct qht_map - structure to track an array of buckets
-- 
2.10.1