Re: [RFC v1 2/2] mm: SLUB Freelist randomization

[Thomas Garnier]

On Thu, May 19, 2016 at 7:15 PM, Joonsoo Kim <js1304@xxxxxxxxx> wrote:
> 2016-05-20 5:20 GMT+09:00 Thomas Garnier <thgarnie@xxxxxxxxxx>:
>> I ran the test given by Joonsoo and it gave me these minimum cycles
>> per size across 20 usage:
>
> I can't understand what you did here. Maybe, it's due to my poor Engling.
> Please explain more. You did single thread test? Why minimum cycles
> rather than average?
>

I used your version of slab_test and ran it 20 times for each
versions. I compared
the minimum number of cycles as an optimal case for comparison. As you said
slab_test results can be unreliable. Comparing the average across multiple runs
always gave odd results.

>> size,before,after
>> 8,63.00,64.50 (102.38%)
>> 16,64.50,65.00 (100.78%)
>> 32,65.00,65.00 (100.00%)
>> 64,66.00,65.00 (98.48%)
>> 128,66.00,65.00 (98.48%)
>> 256,64.00,64.00 (100.00%)
>> 512,65.00,66.00 (101.54%)
>> 1024,68.00,64.00 (94.12%)
>> 2048,66.00,65.00 (98.48%)
>> 4096,66.00,66.00 (100.00%)
>
> It looks like performance of all size classes are the same?
>
>> I assume the difference is bigger if you don't have RDRAND support.
>
> What does RDRAND means? Kconfig? How can I check if I have RDRAND?
>

Sorry, I was referring to the usage of get_random_bytes_arch which
will be faster
if the test machine support specific instructions (like RDRAND).

>> Christoph, Joonsoo: Do you think it would be valuable to add a CONFIG
>> to disable additional randomization per new page? It will remove
>> additional entropy but increase performance for machines without arch
>> specific randomization instructions.
>
> I don't think that it deserve another CONFIG. If performance is a matter,
> I think that removing additional entropy is better until it is proved that
> entropy is a problem.
>

I will do more testing before the next RFC to decide the best approach.

> Thanks.

Thanks for the comments,
Thomas