Re: [PATCH] reorder struct files_struct

[Dipankar Sarma <dipankar@in.ibm.com>]

On Wed, Sep 14, 2005 at 11:17:42PM +0200, Eric Dumazet wrote:
> In SMP (and NUMA) environnements, each time a thread wants to open or close 
> a file, it has to acquire the spinlock, thus invalidating the cache line 
> containing this spinlock on other CPUS. So other threads doing 
> read()/write()/... calls that use RCU to access the file table are going to 
> ask further memory (possibly NUMA) transactions to read again this memory 
> line.
> 
> Please consider applying this patch. It moves the spinlock to another cache 
> line, so that concurrent threads can share the cache line containing 
> 'count' and 'fdt' fields.
> 
> --- linux-2.6.14-rc1/include/linux/file.h	2005-09-13 05:12:09.000000000 +0200
> +++ linux-2.6.14-rc1-ed/include/linux/file.h	2005-09-15 01:09:13.000000000 +0200
> @@ -34,12 +34,12 @@
>   */
>  struct files_struct {
>          atomic_t count;
> -        spinlock_t file_lock;     /* Protects all the below members.  Nests inside tsk->alloc_lock */
>  	struct fdtable *fdt;
>  	struct fdtable fdtab;
>          fd_set close_on_exec_init;
>          fd_set open_fds_init;
>          struct file * fd_array[NR_OPEN_DEFAULT];
> +	spinlock_t file_lock;     /* Protects concurrent writers.  Nests inside tsk->alloc_lock */
>  };
>  
>  #define files_fdtable(files) (rcu_dereference((files)->fdt))

For most apps without too many open fds, the embedded fd_sets
are going to be used. Wouldn't that mean that open()/close() will
invalidate the cache line containing fdt, fdtab by updating
the fd_sets ? If so, you optimization really doesn't help.


Thanks
Dipankar