On 2/2/16 12:23 PM, Xin LI wrote:


On Tuesday, February 2, 2016, Alfred Perlstein <alf...@freebsd.org <mailto:alf...@freebsd.org>> wrote:



    On 2/2/16 11:39 AM, John Baldwin wrote:

        On Tuesday, February 02, 2016 05:57:59 AM Alfred Perlstein wrote:

            Author: alfred
            Date: Tue Feb  2 05:57:59 2016
            New Revision: 295136
            URL: https://svnweb.freebsd.org/changeset/base/295136

            Log:
               Increase max allowed backlog for listen sockets
               from short to int.
                  PR: 203922
               Submitted by: White Knight <white_kni...@2ch.net>
               MFC After: 4 weeks

        You do realize that this breaks the ABI of the sysctls used to
        fetch
        connection lists (and so will break existing binaries like
        ucd-snmpd, etc.)
        and thus can't be MFC'd right?

    OK, I will not MFC it.

    Is it worthwhile to extend the xsocket to have padding so that in
    11.x and beyond we can allow for some changes in the structure?

    Another idea I had was to include a version number with the sysctl
    request so that we can send back versioned structures, let me know
    what you think about that.

    The first idea will take not more than a few days for me to
    accomplish, the second (versioning the sysctl) probably a few more
    days than that.


We have similar construction (versioned ioctl) in FreeBSD ZFS but the main goal is to keep system bootable, not to support all functionalities. Do we change the structure often and is it important enough to warrant the complexity? I think kmem interface have a simple size check to guard against world/kernel inconsistency.

I would second John's comment on the necessity of the change though, if one already have 32K of *backlogged* connections, it's probably not very useful to allow more coming in. It sounds like the application itself is seriously broken, and unless expanding the field have some performance benefit, I don't think it should stay.

Imagine a hugely busy image board like 2ch.net, if there is a single hiccup, it's very possible to start dropping connections.

I stand by the scalability improvement offered here even though it is an edge case. Linux appears to offer 32 bits of backlog and so should we.

-Alfred
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