netdev - Re: [RFC net-next 2/2] udp: No longer use SLAB_DESTROY_BY

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Message-ID: <1459191346.6473.111.camel@edumazet-glaptop3.roam.corp.google.com>
Date:	Mon, 28 Mar 2016 11:55:46 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Rick Jones <rick.jones2@....com>
Cc:	Eric Dumazet <edumazet@...gle.com>,
	"David S . Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Tom Herbert <tom@...bertland.com>
Subject: Re: [RFC net-next 2/2] udp: No longer use SLAB_DESTROY_BY_RCU

On Mon, 2016-03-28 at 11:44 -0700, Rick Jones wrote:
> On 03/28/2016 10:00 AM, Eric Dumazet wrote:
> > On Mon, 2016-03-28 at 09:15 -0700, Rick Jones wrote:
> >> On 03/25/2016 03:29 PM, Eric Dumazet wrote:
> >>> UDP sockets are not short lived in the high usage case, so the added
> >>> cost of call_rcu() should not be a concern.
> >>
> >> Even a busy DNS resolver?
> >
> > If you mean that a busy DNS resolver spends _most_ of its time doing :
> >
> > fd = socket()
> > bind(fd  port=0)
> >    < send and receive one frame >
> > close(fd)
> 
> Yes.  Although it has been a long time, I thought that say the likes of 
> a caching named in the middle between hosts and the rest of the DNS 
> would behave that way as it was looking-up names on behalf those who 
> asked it.

I really doubt a modern program would dynamically allocate one UDP port
for every in-flight request, as it would limit them to number of
ephemeral ports concurrent requests (~30000 assuming the process can get
them all on the host)

Managing a pool would be more efficient (The 1.3 usec penalty becomes
more like 4 usec in multi threaded programs)

Sure, you always can find badly written programs, but they already hit
scalability issues anyway.

UDP refcounting cost about 2 cache line misses per packet in stress
situations, this really has to go, so that well written programs can get
full speed.