Re: [patch v2 4/5] percpu_counter: use atomic64 for counter in SMP

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 11 May 2011 16:10:16 +0800 Shaohua Li <shaohua.li@intel.com> wrote:

> The percpu_counter global lock is only used to protect updating fbc->count after
> we use lglock to protect percpu data. Uses atomic64 for percpu_counter, because
> it is cheaper than spinlock. This doesn't slow fast path (percpu_counter_read).
> atomic64_read equals to read fbc->count for 64-bit system, or equals to
> spin_lock-read-spin_unlock for 32-bit system.
> 
> Note, originally the percpu_counter_read for 32-bit system doesn't hold
> spin_lock, but that is buggy and might cause very wrong value accessed.
> This patch fixes the issue.
> 
> This can also improve some workloads with percpu_counter->lock heavily
> contented. For example, vm_committed_as sometimes causes the contention.
> We should tune the batch count, but if we can make percpu_counter better,
> why not? In a 24 CPUs system and 24 processes, each runs:
> while (1) {
> 	mmap(128M);
> 	munmap(128M);
> }
> we then measure how many loops each process can take:
> orig: 1226976
> patched: 6727264
> The atomic method gives 5x~6x faster.

How much slower did percpu_counter_sum() become?