Message-ID: <42422165.20505@yahoo.com.au>
Date: Thu, 24 Mar 2005 13:09:41 +1100
From: Nick Piggin <nickpiggin@yahoo.com.au>
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To: Blaisorblade <blaisorblade@yahoo.it>
CC: user-mode-linux-devel@lists.sourceforge.net,
       Bodo Stroesser <bstroesser@fujitsu-siemens.com>, akpm@osdl.org,
       jdike@addtoit.com, linux-kernel@vger.kernel.org
Subject: Re: [uml-devel] [patch 02/12] uml: cpu_relax fix
References: <20050322162121.4295D2125C@zion> <4241A2C0.2050206@fujitsu-siemens.com> <200503240250.38153.blaisorblade@yahoo.it>
In-Reply-To: <200503240250.38153.blaisorblade@yahoo.it>
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Blaisorblade wrote:
> On Wednesday 23 March 2005 18:09, Bodo Stroesser wrote:
> 
>>blaisorblade@yahoo.it wrote:
>>
>>>Use rep_nop instead of barrier for cpu_relax, following $(SUBARCH)'s
>>>doing that (i.e. i386 and x86_64).
>>
>>IIRC, Jeff had the idea, to use sched_yield() for this (from a discussion
>>on #uml).
> 
> Hmm, makes sense, but this is to benchmark well... I remember from early 
> discussions on 2.6 scheduler that using sched_yield might decrease 
> performance (IIRC starve the calling application).
> 

Typically, for places where cpu_relax is used, sched_yield would be
a poor fit. So yes it could easily reduce performance.

> Also, that call should be put inside the idle loop, not for cpu_relax, which 
> is very different, since it is used (for instance) in kernel/spinlock.c for 
> spinlocks, and in such things. The "Pause" opcode is explicitly recommended 
> (by Intel manuals, I don't recall why) for things like spinlock loops, and 
> using yield there would be bad.
> 

The other thing is that sched_yield won't relax at all if you are the
only thing running, it will be a busy wait. So again, maybe not a great
fit for the idle loop either.


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