Date: Wed, 4 Sep 2002 09:42:44 -0400 (EDT) From: Andrew Gallatin <gallatin@cs.duke.edu> To: John Baldwin <jhb@FreeBSD.ORG> Cc: alpha@FreeBSD.ORG, peter@FreeBSD.ORG Subject: Re: FYI: Buffer cache fix gave major speedup Message-ID: <15734.3540.941843.110159@grasshopper.cs.duke.edu> In-Reply-To: <XFMail.20020904090455.jhb@FreeBSD.org> References: <XFMail.20020904090455.jhb@FreeBSD.org>
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John Baldwin writes:
> On the DS20 I have here, Peter's fix to basically enable the buffer cache
> gave me about a 38% performance increase for a buildworld -j 2 on current:
>
> Before:
> --------------------------------------
> build started at 15:11:08 on 08/28/02
> build finished at 18:43:05 on 08/28/02
> --------------------------------------
> Which is a total time of 3:31:57
>
> After:
> --------------------------------------
> build started at 22:41:31 on 09/03/02
> build finished at 00:51:46 on 09/04/02
> --------------------------------------
> Which is a total time of 2:10:15
>
> If this bug exists in 4.x we should really fix it there as well.
>
Are you talking about btoc()? I think stable is immune. A simple
test shows the buffer cache is working just fine there.
The problem was that phk's version of btoc was truncating
things by using a 32-bit cast. Eg, phk replaced this:
#define btoc(x) (((x) + PAGE_MASK) >> PAGE_SHIFT)
With this:
#define btoc(x) (((unsigned)(x)+PAGE_MASK)>>PAGE_SHIFT)
^^^^^^^^^
Which Peter replaced with:
#define btoc(x) (((vm_offset_t)(x)+PAGE_MASK)>>PAGE_SHIFT)
So the difference between -stable and -current is now just the
(vm_offset_t) cast. I don't _think_ that should make a difference.
But I'm afraid I don't understand how the cast caused such an
innocuous problem as disabling the buffer cache. I'd have thought it
would have caused memory corruption.
Drew
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