Date: Wed, 10 Feb 2016 10:08:48 -0500 From: Pedro Giffuni <pfg@FreeBSD.org> To: Hans Petter Selasky <hps@selasky.org>, Ryan Stone <rysto32@gmail.com> Cc: "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: <56BB5280.5060609@FreeBSD.org> In-Reply-To: <56BAE4BC.9000105@selasky.org> References: <201601191533.u0JFXSxf037804@repo.freebsd.org> <CAFMmRNz3uXim3H3-sGuBUBs45Jy8p260ywothgp4iFkUcnvnEw@mail.gmail.com> <56BAE4BC.9000105@selasky.org>
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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). Pedro.
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