On Tue, 27 Oct 2015 12:15:16 +0100 Hannes Frederic Sowa <han...@stressinduktion.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 -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
