linux-kernel - Re: Can context switches be faster?

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Message-ID: <452EA700.9060009@goop.org>
Date:	Thu, 12 Oct 2006 13:35:12 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	John Richard Moser <nigelenki@...cast.net>
CC:	Chris Friesen <cfriesen@...tel.com>, linux-kernel@...r.kernel.org
Subject: Re: Can context switches be faster?

John Richard Moser wrote:
> That's a load more descriptive :D
>
> 0.890 uS, 0.556uS/cycle, that's barely 2 cycles you know.  (Pentium M)
> PPC performs similarly, 1 cycle should be about 1uS.
>   

No, you're a factor of 1000 off - these numbers show the context switch 
is around 1600-75000 cycles.  And that doesn't really tell the whole 
story: if caches/TLB get flushed on context switch, then the newly 
switched-to task will bear the cost of having cold caches, which isn't 
visible in the raw context switch time.

But modern x86 processors have a very quick context switch time, and I 
don't think there's much room for improvement aside from 
micro-optimisations (though that might change if the architecture grows 
a way to avoid flushing the TLB on switch).

    J
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