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Message-ID: <20151027130423.63f24dbb@redhat.com>
Date: Tue, 27 Oct 2015 13:04:23 +0100
From: Jesper Dangaard Brouer <brouer@...hat.com>
To: Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc: brouer@...hat.com, Richard Cochran <richardcochran@...il.com>,
netdev@...r.kernel.org
Subject: Re: [PATCH net-next] sock: don't enable netstamp for af_unix
sockets
On Tue, 27 Oct 2015 12:15:16 +0100 Hannes Frederic Sowa <hannes@...essinduktion.org> wrote:
> On Tue, Oct 27, 2015, at 12:09, Hannes Frederic Sowa wrote:
> > Hi Richard,
> >
> > On Tue, Oct 27, 2015, at 11:11, Richard Cochran wrote:
> > > On Mon, Oct 26, 2015 at 02:32:59PM +0100, Hannes Frederic Sowa wrote:
> > > > On Mon, Oct 26, 2015, at 14:19, Richard Cochran wrote:
> > > > > On Mon, Oct 26, 2015 at 01:51:37PM +0100, Hannes Frederic Sowa wrote:
> > > > > > netstamp_needed is toggled for all socket families if they request
> > > > > > timestamping. But some protocols don't need the lower-layer timestamping
> > > > > > code at all. This patch starts disabling it for af-unix.
> > > > >
> > > > > What problem is this patch trying to solve?
> > > >
> > > > netstamp_needed is a static-key which enables timestamping code in the
> > > > networking stack receive functions for every packet, while it is not
> > > > needed for AF_UNIX/LOCAL. So it is merely a small performance
> > > > enhancement.
> > >
> > > Are there any numbers that show the effect of this enhancement?
> >
> > I haven't personally done any performance numbers.
> >
> > Jesper (in Cc) noticed that it showed up in perf performance reports
> > even though he used a very minimal setup. Turned out that
> > systemd-journald enables timestamping on AF_UNIX sockets which thus
> > enabled netstamps globally. I think Jesper can chime in here.
Well, it should be quite obvious that requesting a timestamp on every
packet is a fairly expensive, especially when not used for anything.
I can estimate the cost by looking at perf report, on a single-flow
IP-fwd test (1989575 pps) CPU i7-4790K @ 4.2GHz.
I quick IP-fwd test show perf top:
1.54% ksoftirqd/1 [kernel.vmlinux] [k] read_tsc
1.07% ksoftirqd/1 [kernel.vmlinux] [k] ktime_get_with_offset
(1/1989575*10^9)*((1.54+1.07)/100) = 13.12 nanosec
On some of my slower systems, I've seen cost of just reading TSC be
around 32 ns.
> Also counter question: why is the netstamp code protected by a
> static_key otherwise if not for trying to suppress the code path as
> often as possible if not used? ;)
Exactly ;-)
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
Author of http://www.iptv-analyzer.org
LinkedIn: http://www.linkedin.com/in/brouer
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