From owner-freebsd-current  Mon Jan 21  2: 2:27 2002
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Date: Mon, 21 Jan 2002 02:02:17 -0800 (PST)
From: Raman Ng <ramanng@yahoo.com>
Reply-To: raman@hello.to
Subject: Re: Extended paging bug in Athlon affect FreeBSD?
To: Terry Lambert <tlambert2@mindspring.com>, raman@hello.to
Cc: stable@freebsd.org, current@freebsd.org
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> I am well aware of this bug.
> 
> It does not affect FreeBSD, which only uses 4M pages
> for
> the first 4M of the kernel itself.
> 
> I've worked on code that enables 4M pages on other
> memory
> used in FreeBSD, that had this problem, but only if
> you
> were really stupid in your allocation mechanism.
> 
> There's a workaround for this problem which is
> fairly
> trivial to implement in software, and should
> probably be
> done when 4M pages are enabled, if you are using an
> Athlon,
> and are adding 4M pages.  The problem didn't bite my
> own
> application, since I knew about it and used the TLBs
> in a
> very specific way to get around the problem, but I'm
> told
> that after I left the company, someone went in and
> used
> them a different way, not knowing about the problem
> or the
> workaround, and broke things.  This resulted in
> about a
> 14% performance loss.
> 
> If you are doing heavy networking on a machine with
> a lot
> of memory, it's reasonable to move your mbufs into
> 4M pages,
> which nets you a 4-14% improvement (depending on
> whether or
> not you also change your allocator: 14% if you do). 
> If you
> do that, knowing how to work around the problem nets
> you a
> pretty big win in overall performance, for about
> 6-10 more
> machine instructions at startup time.  8-).
> 
> In any case, this will not be a problem for FreeBSD,
> and is
> only a problem for Linux because of the strange way
> they
> initialize things.
> 
> -- Terry

Glad to know FreeBSD is not affected.  Thanks a lot.

Cheers,
Raman

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