lkml.org 
[lkml]   [2005]   [Sep]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] reorder struct files_struct
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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-09-15 00:44    [W:0.062 / U:0.248 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site