Ravikiran G Thirumalai a écrit :
The net_device has a refcnt used to keep track of it's uses.
This is used at the time of unregistering the network device
(module unloading ..) (see netdev_wait_allrefs) .
For loopback_dev , this refcnt increment/decrement is causing
unnecessary traffic on the interlink for NUMA system
affecting it's performance. This patch improves tbench numbers by 6% on a
8way x86 Xeon (x445).
===================================================================
--- alloc_percpu-2.6.13.orig/include/linux/netdevice.h 2005-08-28 16:41:01.000000000 -0700
+++ alloc_percpu-2.6.13/include/linux/netdevice.h 2005-09-12 11:54:21.000000000 -0700
@@ -37,6 +37,7 @@
#include <linux/config.h>
#include <linux/device.h>
#include <linux/percpu.h>
+#include <linux/bigref.h>
struct divert_blk;
struct vlan_group;
@@ -377,7 +378,7 @@
/* device queue lock */
spinlock_t queue_lock;
/* Number of references to this device */
- atomic_t refcnt;
+ struct bigref netdev_refcnt;
/* delayed register/unregister */
struct list_head todo_list;
/* device name hash chain */
@@ -677,11 +678,11 @@
Hum...
Did you tried to place refcnt/netdev_refcnt in a separate cache line than
queue_lock ? I got good results too...
> /* device queue lock */
> spinlock_t queue_lock;
> /* Number of references to this device */
> - atomic_t refcnt;
> + struct bigref netdev_refcnt ____cacheline_aligned_in_smp ;
> /* delayed register/unregister */
> struct list_head todo_list;
> /* device name hash chain */
Every time a cpu take the queue_lock spinlock, it exclusively gets one cache
line. If another cpu try to access netdev_refcnt, it has to grab this cache
line (even if properely per_cpu designed, there is still one shared field). In
fact the whole struct net_device should be re-ordered for SMP/NUMA performance.
Eric
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