netdev - Re: [PATCH v5 net-next] tcp: switch rtt estimations to usec resolution

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Date:	Mon, 24 Feb 2014 17:38:59 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Stephen Hemminger <stephen@...workplumber.org>
Cc:	Julian Anastasov <ja@....bg>, David Miller <davem@...emloft.net>,
	Yuchung Cheng <ycheng@...gle.com>,
	netdev <netdev@...r.kernel.org>,
	Neal Cardwell <ncardwell@...gle.com>,
	Larry Brakmo <brakmo@...gle.com>
Subject: Re: [PATCH v5 net-next] tcp: switch rtt estimations to usec
 resolution

On Mon, 2014-02-24 at 17:11 -0800, Eric Dumazet wrote:
> On Mon, 2014-02-24 at 15:51 -0800, Stephen Hemminger wrote:
> 
> > 
> > The original reason I kept ms resolution and added the flag was
> > that accessing high resolution time is expensive for systems where
> > TSC is not stable.  Google may live in a world of SMP systems with
> > good X86 CPUs with working TSC, but other architectures and system
> > config's may have non-working TSC.
> 
> Well, it seems we now have alternatives to ktime_get() that were
> probably not yet there at the time you made this choice ?
> 
> We only want a reasonable alternative to jiffies based measures,
> we do not need ultra precise and synchronized clocks.
> 
> I'll redo tests with hpet clocksource, and local_clock()

Interesting : Using current net-next kernel, before my patches I get
on a TCP_RR workload :

It seems nobody is seriously using hpet these days with recent kernels.


    29.98%  [kernel]      [k] read_hpet                     
     2.93%  [kernel]      [k] _raw_spin_lock                
     1.63%  [kernel]      [k] __schedule                    
     1.62%  [bnx2x]       [k] bnx2x_start_xmit              
     1.57%  [kernel]      [k] __netif_receive_skb_core      
     1.45%  [kernel]      [k] tcp_ack                       
     1.30%  [kernel]      [k] idle_cpu                      
     1.29%  [kernel]      [k] __switch_to                   
     1.28%  netperf       [.] send_omni_inner               
     1.05%  libc-2.15.so  [.] __libc_recv                   
     0.98%  [kernel]      [k] ipv4_dst_check                



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