Date: Sat, 13 Feb 2016 08:47:53 +0100 From: Hans Petter Selasky <hps@selasky.org> To: Warner Losh <imp@bsdimp.com>, Pedro Giffuni <pfg@freebsd.org> Cc: Ryan Stone <rysto32@gmail.com>, "src-committers@freebsd.org" <src-committers@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "svn-src-head@freebsd.org" <svn-src-head@freebsd.org> Subject: Re: svn commit: r294327 - in head/sys: dev/cxgb dev/cxgbe dev/e1000 dev/hyperv/netvsc dev/ixgbe dev/mxge netinet sys Message-ID: <56BEDFA9.9080305@selasky.org> In-Reply-To: <CANCZdfpmYMGKJkX4uQR6Uet18cffJ4-uXxRSxfbe3Q6p2Pb48Q@mail.gmail.com> References: <201601191533.u0JFXSxf037804@repo.freebsd.org> <CAFMmRNz3uXim3H3-sGuBUBs45Jy8p260ywothgp4iFkUcnvnEw@mail.gmail.com> <56BAE4BC.9000105@selasky.org> <56BB5280.5060609@FreeBSD.org> <CANCZdfpmYMGKJkX4uQR6Uet18cffJ4-uXxRSxfbe3Q6p2Pb48Q@mail.gmail.com>
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On 02/11/16 17:47, Warner Losh wrote: > On Wed, Feb 10, 2016 at 8:08 AM, Pedro Giffuni <pfg@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@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? > No, I've abandoned the mergesort patch. --HPS
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