Re: [PATCH 4/4]percpu_counter: use atomic64 for counter

[Shaohua Li <shaohua.li@intel.com>]

On Wed, 2011-04-13 at 03:15 +0800, Tejun Heo wrote:
> Hello,
> 
> On Tue, Apr 12, 2011 at 04:04:06PM +0800, Shaohua Li wrote:
> > should tunning the batch count, but if we can make percpu_counter better, why
> > not?
> 
> First of all, the lock being only in slow paths, it's quite unlikely
> to get ever contended.  Also, because the lock duration is always
> extremely short, conversion to atomic_t isn't too likely to gain
> anything significant, especially in cold paths.
> 
> That said, if it's all gains, why not?  I don't know.  Maybe.  Given
> problems in the previous patches, I don't feel too enthusiastic for
> this series at this point but I'm quite sleep deprived now so it might
> just be me not the patch itself.
yes, we do have something this matters. the vm_committed_as
percpu_counter could heavily contend the lock. please see
http://marc.info/?l=linux-kernel&m=130144786026044&w=2
We should fix vm_committed_as, but the atomic method also gives me very
good output. When I do mmap/munmap test in a 24 CPU system, the atomic
method gives me 50x faster.

Thanks,
Shaohua