Messages in this thread | | | Date | Thu, 15 Sep 2005 03:32:05 +0530 | From | Dipankar Sarma <> | Subject | Re: [PATCH] reorder struct files_struct |
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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
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