linux-kernel - Re: Unix sockets via TCP on localhost: is TCP slower?

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Message-ID: <3aaafc130811140514u4e290b41t78da818d304fce86@mail.gmail.com>
Date:	Fri, 14 Nov 2008 08:14:22 -0500
From:	"J.R. Mauro" <jrm8005@...il.com>
To:	"Olaf van der Spek" <olafvdspek@...il.com>
Cc:	"Eric Dumazet" <dada1@...mosbay.com>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: Unix sockets via TCP on localhost: is TCP slower?

On Fri, Nov 14, 2008 at 4:06 AM, Olaf van der Spek <olafvdspek@...il.com> wrote:
> On Fri, Nov 14, 2008 at 9:54 AM, Eric Dumazet <dada1@...mosbay.com> wrote:
>>> I expected the kernel to copy data directly from user-space of the
>>> sending process to a kernel buffer of the receiving process, much like
>>> UNIX sockets.
>>>
>>
>> localhost uses a standard network device, and whole network stack
>> is used, no 'special kludges'. You can add iptables rules, you
>> can do trafic shaping, trafic sniffing (tcpdump), limiting
>> memory used by all sockets (controlling memory pressure on the
>> machine)
>>
>> Doing what you suggest would slow down AF_INET stack.
>
> Why?

Because then the AF_INET stack would have to check *every* time
something went through it and see if it's bound for localhost. You're
adding more complexity to the stack just to make the time on 1 case
speed up, but you're slowing down every single other case.

>
>> You probably can expect AF_UNIX to be faster, since this one is really
>> special and use shortcuts.
>>
>> Then, you probably can use shared memory instead of AF_UNIX, or
>> pipes (and splice()), or ...
>>
>> Then you probably can use threads and do zero-copy ;)
>
> Hmm, I'd like to avoid running my web server inside of my database
> server process. ;)
>
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