netdev - Re: Same data to several sockets with just one syscall ?

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Message-ID: <1455560180.11190.12.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Mon, 15 Feb 2016 19:16:20 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Claudio Scordino <claudio@...dence.eu.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Same data to several sockets with just one syscall ?

On Mon, 2016-02-15 at 11:03 +0100, Claudio Scordino wrote:
> Hi Eric,
> 
> 2016-02-12 11:35 GMT+01:00 Eric Dumazet <eric.dumazet@...il.com>:
> > On Fri, 2016-02-12 at 09:53 +0100, Claudio Scordino wrote:
> >
> >> This makes the application waste time in entering/exiting the kernel
> >> level several times.
> >
> > syscall overhead is usually small. Real cost is actually getting to the
> > socket objects (fd manipulation), that you wont avoid with a
> > super-syscall anyway.
> 
> Thank you for answering. I see your point.
> 
> However, assuming that a switch from user-space to kernel-space (and
> back) needs about 200nsec of computation (which I guess is a
> reasonable value for a 3GHz x86 architecture), the 50th receiver
> experiences a latency of about 10 usec. In some domains (e.g.,
> finance) this delay is not negligible.

I thought these domains were using multicast.

> 
> Moving the "fan-out" code into kernel space would remove this waste of
> time. IMHO, the latency reduction would pay back the 100 lines of code
> for adding a new syscall.

It wont reduce the latency at all, and add a lot of maintenance hassle.

syscall overhead is about 40 ns.
This is the time taken to transmit ~50 bytes on 10Gbit link.

40ns * 50 = 2 usec only.

Feel free to implement your idea and test it, you'll discover the added
complexity is not worth it.