Messages in this thread | | | Date | Thu, 15 Sep 2005 00:42:03 +0200 | From | Eric Dumazet <> | Subject | Re: [PATCH] reorder struct files_struct |
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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.
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