Re: [PATCH] slub: reduce overhead of slub_debug

[David Miller <davem@davemloft.net>]

From: Christoph Lameter <cl@linux.com>
Date: Thu, 7 Jul 2011 14:12:37 -0500 (CDT)

> On Thu, 7 Jul 2011, Pekka Enberg wrote:
> 
>> On Thu, 7 Jul 2011, Pekka Enberg wrote:
>> > > Looks good to me. Christoph, David, ?
>>
>> On Thu, 2011-07-07 at 13:17 -0500, Christoph Lameter wrote:
>> > The reason debug code is there is because it is useless overhead typically
>> > not needed. There is no point in optimizing the code that is not run in
>> > production environments unless there are gross performance issues that
>> > make debugging difficult. A performance patch for debugging would have to
>> > cause significant performance improvements. This patch does not do that
>> > nor was there such an issue to be addressed in the first place.
>>
>> Is there something technically wrong with the patch? Quoting the patch
>> email:
>>
>>   (Compiling some project with different options)
>>                                  make -j12    make clean
>>   slub_debug disabled:             1m 27s       1.2 s
>>   slub_debug enabled:              1m 46s       7.6 s
>>   slub_debug enabled + this patch: 1m 33s       3.2 s
>>
>>   check_bytes still shows up high, but not always at the top.
>>
>> That's significant enough speedup for me!
> 
> Ok. I had a different set of numbers in mind from earlier posts.
> 
> The benefit here comes from accessing memory in larger (word) chunks
> instead of byte wise. This is a form of memscan() with inverse matching.
> 
> Isnt there an asm optimized version that can do this much better (there is
> one for memscan())? Optimizing this in core code by codeing something as
> generic as that is not that good since the arch code can deliver better
> performance and it seems that this is functionality that could be useful
> elsewhere.

You're being so unreasonable, just let the optimization in, refine it
with follow-on patches.