Messages in this thread | | | Subject | Re: [PATCH 16/17] fs: Convert nr_inodes to a per-cpu counter | From | Eric Dumazet <> | Date | Sat, 16 Oct 2010 11:31:15 +0200 |
| |
Le samedi 16 octobre 2010 à 02:07 -0700, Andrew Morton a écrit : > On Sat, 16 Oct 2010 10:29:08 +0200 Eric Dumazet <eric.dumazet@gmail.com> 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-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/
| |