svn commit: r294327 - in head/sys: dev/cxgb dev/cxgbe dev/e1000 dev/hyperv/netvsc dev/ixgbe dev/mxge netinet sys

[Warner Losh]

On Wed, Feb 10, 2016 at 8:08 AM, Pedro Giffuni <pfg at freebsd.org> wrote:

> Hello;
>
> El 10/02/2016 a las 02:20, Hans Petter Selasky escribió:
>
>> On 01/19/16 17:09, Ryan Stone wrote:
>>
>>> On Tue, Jan 19, 2016 at 10:33 AM, Hans Petter Selasky <
>>> hselasky at freebsd.org>
>>> wrote:
>>>
>>>
>>>> +       qsort(lc->lro_mbuf_data, lc->lro_mbuf_count, sizeof(struct mbuf
>>>> *),
>>>> +           &tcp_lro_mbuf_compare_header);
>>>>
>>>>
>>> In the worst case, qsort() can take O(n**2) time and consume O(n) stack
>>> space.  Is there a DOS concern here?
>>>
>>>
>> Hi,
>>
>> Our FreeBSD qsort() routine has been specifically modified to not exhibit
>> the so-called QuickSort worst case behaviour of O(N**2) sorting time. This
>> is not documented in our source code, but here:
>>
>> http://cs.fit.edu/~pkc/classes/writing/samples/bentley93engineering.pdf
>>
>> So I think DOS w.r.t O(N**2) is not a valid consern.
>>
>> Thank you for your input Ryan.
>>
>> BTW:
>>
>> Drew Gallatin has tested our qsort() v.s. my mergesort() and found that:
>>
>> "It looks like mergesort is nearly 2x as expensive. (4.7% vs 2.5%)"
>>
>>
> FWIW, our libc qsort() has an additional enhancement:
>
> http://svnweb.freebsd.org/base?view=revision&revision=279663
>
> In my measurements qsort(3) was now always faster than mergesort(3).


If it is faster, is there any good reason to maintain both qsort and
mergesort
in the kernel then?

Warner