Re: [PATCH] reorder struct files_struct

[Eric Dumazet]

Dipankar Sarma a écrit :
> 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.

If the embedded struct fdtable is used, then the only touched field is 
'next_fd', so we could also move this field at the end of 'struct fdtable'

But I wonder if 'next_fd' really has to be in 'struct fdtable', maybe it could 
be moved to 'struct files_struct' close to file_lock ?

If yes, the whole embedded struct fdtable is readonly.

Eric
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