Le samedi 16 octobre 2010 Ã 02:07 -0700, Andrew Morton a Ãcrit : > On Sat, 16 Oct 2010 10:29:08 +0200 Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote: > > Some people believe percpu_counter object is the right answer to such > > distributed counters, because the loop is done on 'online' cpus instead > > of 'possible' cpus. "It must be better if number of possible cpus is > > 4096 and only one or two cpus are online"... > > > > But if we do this loop only on rare events, like > > "cat /proc/sys/fs/inode-nr", then the percpu_counter() is more > > expensive, because percpu_add() _is_ more expensive : > > > > - Its a function call and lot of instructions/cycles per call, while > > this_cpu_inc(nr_inodes) is a single instruction, using no register on > > x86. > > You want an inlined percpu_counter_inc() then write one! Bonus points > for writing this_cpu_add_return() and doing it without a > preempt_disable(). It collapses to just a few instructions. > A few instructions, but no guarantee of false sharing eviction. Each time one cpu dirties the percpu_counter object, it slow down other cpus because they need to fetch the cache line again. Btw, I believe my previous patch against include/linux/percpu_counter.h was lost. Are you sure I am the right guy to work on percpu_counter infra ? If yes I can implement your inlined idea. Thanks -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
