From: "Albert D. Cahalan" <acahalan@cs.uml.edu>
Message-Id: <200204232242.g3NMgaW215059@saturn.cs.uml.edu>
Subject: Re: Why HZ on i386 is 100 ?
To: matti.aarnio@zmailer.org (Matti Aarnio)
Date: Tue, 23 Apr 2002 18:42:35 -0400 (EDT)
Cc: rml@tech9.net (Robert Love), torvalds@transmeta.com (Linus Torvalds),
        mark@mark.mielke.cc (Mark Mielke), davidm@hpl.hp.com,
        davidel@xmailserver.org (Davide Libenzi), linux-kernel@vger.kernel.org
In-Reply-To: <20020417110440.G12961@mea-ext.zmailer.org> from "Matti Aarnio" at Apr 17, 2002 11:04:40 AM
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Matti Aarnio writes:
> On Wed, Apr 17, 2002 at 02:01:42AM -0400, Robert Love wrote:
>> On Wed, 2002-04-17 at 01:34, Linus Torvalds wrote:

>>> No, it also makes it much easier to convert to/from the standard UNIX time
>>> formats (ie "struct timeval" and "struct timespec") without any surprises,
>>> because a jiffy is exactly representable in both if you have a HZ value
>>> of 100 or 100, but not if your HZ is 1024.
>>
>> Exactly - this was my issue.  So what _was_ the rationale behind Alpha
>> picking 1024 (and others following)?  More importantly, can we change to
>> 1000?
>
> Alpha processors don't have full division hardware, they have to
> iterate it one bit at the time. They do have a flash multiplier,
> and a barrel-shifter.  Shifts take one pipeline cycle, like to
> addition and substraction.  Multiply takes 6-12 depending on model,
> but division takes 64...

Division by 1000 is a UMULH followed by a right shift.
So maybe it costs you one cycle more than division by 1024 would.
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