linux-kernel - Re: 2048 CPUs [was: Re: New filesystem for Linux]

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Message-ID: <Pine.LNX.4.64.0611122103060.4965@artax.karlin.mff.cuni.cz>
Date:	Sun, 12 Nov 2006 21:07:25 +0100 (CET)
From:	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>
To:	Pavel Machek <pavel@....cz>
Cc:	Albert Cahalan <acahalan@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: 2048 CPUs [was: Re: New filesystem for Linux]

Hi!

> Hi!
>
>>>> You can't tell that CPUs behave exactly
>>>> probabilistically --- it may
>>>> happen that one gets out of the wait loop always too
>>>> late.
>>>
>>> Well,  I don't need them to be _exactly_
>>> probabilistical.
>>>
>>> Anyway, if you have 2048 CPUs... you can perhaps get
>>> some non-broken
>>> ones.
>>
>> No intel document guarantees you that if more CPUs
>> simultaneously execute locked cmpxchg in a loop that a
>
> If we are talking 2048 cpus, we are talking ia64.

IA64 spinlock is locked cmpxchg, if failed than pause (i386 equivalent of 
rep nop) read the value, and if unlocked, try cmpxchg again.

There is no fairness in it.

>> CPU will see compare success in a finite time. In fact,
>> CPUs can't guarantee this at all, because they don't
>> know that they're executing a spinlock --- for them its
>> just an instruction stream like anything else.
>
> ...even i386 has monitor/mwait these days.

It also doesn't guarantee that subsequent locked instruction will take the 
lock after finite number of loops.

Mikulas

> 							Pavel
> -- 
> Thanks for all the (sleeping) penguins.
>
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