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Date:      Wed, 3 Feb 2016 00:41:12 +0300
From:      Slawa Olhovchenkov <slw@zxy.spb.ru>
To:        Alfred Perlstein <alfred@freebsd.org>
Cc:        Xin LI <delphij@gmail.com>, "svn-src-head@freebsd.org" <svn-src-head@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "src-committers@freebsd.org" <src-committers@freebsd.org>, John Baldwin <jhb@freebsd.org>
Subject:   Re: svn commit: r295136 - in head: sys/kern sys/netinet sys/sys usr.bin/netstat
Message-ID:  <20160202214112.GR88527@zxy.spb.ru>
In-Reply-To: <56B11DF0.3060401@freebsd.org>
References:  <201602020557.u125vxCP084718@repo.freebsd.org> <36439709.poT7RgRunK@ralph.baldwin.cx> <56B10D67.4050602@freebsd.org> <CAGMYy3v4s-UQpO3-oq8B1V1NvzyGSmJU49zDUL9oPy21sYkWCQ@mail.gmail.com> <56B11323.70905@freebsd.org> <20160202210958.GV37895@zxy.spb.ru> <56B11DF0.3060401@freebsd.org>

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On Tue, Feb 02, 2016 at 01:21:52PM -0800, Alfred Perlstein wrote:

> >>> 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.
> > In reality start dropping connections in any case: nobody will be
> > infinity wait of accept (user close browser and go away, etc).
> >
> > Also, if you have more then 4K backloged connections -- you have
> > problem, you can't process all connections request and in next second
> > you will be have 8K, after next second -- 12K and etc.
> >


> In our case the user would not really know if our "page" didn't load 
> because we were just an invisible gif.
> 
> So back to the example, let's scale that out to today's numbers.
> 
> 100mbps -> 10gigE, so that would be 1500 conn/sec -> 150,000 conn/sec.  
> so basically at 0.20 of a second of any sort of latency I will be 
> overflowing the listen queue and dropping connections.

OK, you talk about very specilal case -- extremaly short connections,
about one data packet. Yes, in this case you got this behaivor.
I think case of 2ch is different.

> Now when you still have CPU to spare because connections *are* precious, 
> then the model makes sense to slightly over-provision the servers to 
> allow for somebacklog to be processed.
> 
> So, in today's day and age, it really does make sense to allow for 
> buffering more than 32k connections, particularly if the developer knows 
> what he is doing.
> 
> Does this help explain the reasoning?

Yes, some special cases may be exist.



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