Re: [PATCH] percpu_counter: Put a reasonable upper bound on percpu_counter_batch

[Theodore Tso]

On Sep 5, 2011, at 11:48 PM, Tejun Heo wrote:

> On Mon, Aug 29, 2011 at 09:46:09PM +1000, Anton Blanchard wrote:
>> 
>> When testing on a 1024 thread ppc64 box I noticed a large amount of
>> CPU time in ext4 code.
>> 
>> ext4_has_free_blocks has a fast path to avoid summing every free and
>> dirty block per cpu counter, but only if the global count shows more
>> free blocks than the maximum amount that could be stored in all the
>> per cpu counters.
>> 
>> Since percpu_counter_batch scales with num_online_cpus() and the maximum
>> amount in all per cpu counters is percpu_counter_batch * num_online_cpus(),
>> this breakpoint grows at O(n^2).
>> 
>> This issue will also hit with users of percpu_counter_compare which
>> does a similar thing for one percpu counter.
>> 
>> I chose to cap percpu_counter_batch at 1024 as a conservative first
>> step, but we may want to reduce it further based on further benchmarking.
>> 
>> Signed-off-by: Anton Blanchard <anton@xxxxxxxxx>
> 
> Applied to percpu/for-3.2.

Um, this was an ext4 patch and I pointed out it could cause problems.  (Specifically, data loss…)

- Ted

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/