linux-kernel - Re: all syscalls initially taking 4usec on a P4? Re: nonblocking UDPv4 recvfrom() taking 4usec @ 3GHz?

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Date:	Tue, 20 Feb 2007 13:02:26 -0800
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	bert hubert <bert.hubert@...herlabs.nl>
Cc:	Benjamin LaHaise <bcrl@...ck.org>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	Josef Sipek <jsipek@....cs.sunysb.edu>,
	Andi Kleen <andi@...stfloor.org>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: all syscalls initially taking 4usec on a P4? Re: nonblocking
 UDPv4 recvfrom() taking 4usec @ 3GHz?

On Tue, 20 Feb 2007 21:45:05 +0100
bert hubert <bert.hubert@...herlabs.nl> wrote:

> On Tue, Feb 20, 2007 at 02:40:40PM -0500, Benjamin LaHaise wrote:
> 
> > Make sure your system is idle.  Userspace bloat means that *lots* of idle 
> > activity occurs in between timer ticks on recent distributions -- all those 
> 
> You hit the nail on the head. I had previously measured with X shut down,
> but the effect didn't disappear.
> 
> With init=/bin/bash, recvfrom suddenly takes from 900nsec to 1.3usec, with
> only slight correlation between inter-call delay and cycles spent.
> 
> I'm investigating this further as it appears this has a real life effect on
> my P4 - a drastic one!
> 
> processor       : 0
> vendor_id       : GenuineIntel
> cpu family      : 15
> model           : 4
> model name      : Intel(R) Pentium(R) 4 CPU 3.00GHz
> stepping        : 1
> cpu MHz         : 3000.131
> cache size      : 1024 KB
> fdiv_bug        : no
> hlt_bug         : no
> f00f_bug        : no
> coma_bug        : no
> fpu             : yes
> fpu_exception   : yes
> cpuid level     : 5
> wp              : yes
> flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca
> cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx
> constant_tsc pni monitor ds_cpl cid xtpr
> bogomips        : 6003.91
> clflush size    : 64
> 
> Thanks for your help!
> 

What happens with preempt if your process is high priority or SCHED_FIFO/SCHED_RR?

-- 
Stephen Hemminger <shemminger@...ux-foundation.org>
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