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Date: Wed, 10 Nov 2004 20:16:59 -0700
From: Scott Long <scottl@freebsd.org>
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To: David Schultz <das@freebsd.org>
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cc: arch@freebsd.org
Subject: Re: U Area Removal
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David Schultz wrote:
> Over the years, the amount of data we have stored in each process' U
> area has eroded to the point where all we have left are the following:
> 
>   - A struct kinfo_proc that is only used for a.out core dumps.
>     This can be reconstructed at the time of the core dump, so
>     it doesn't need to be there.
> 
>   - The struct pstats for the process, which takes a mere 216 bytes
>     on i386.
> 
> In exchange for the ability to swap out this 216-byte structure, we
> keep around a 4096-byte page, a 132-byte vm_object, and a couple of
> pointers.  Moreover, there is a small amount of runtime overhead
> associated with this, and developers need to remember to PHOLD() and
> PRELE() the process as appropriate.[1]
> 
> I propose to remove the ability to swap the U area, allocating p_stats
> from malloced memory instead.  Medium-term scheduling and swapping of
> kernel stacks would be retained.  Here are the patches; !i386 testers
> wanted:
> 
> 	http://www.freebsd.org/~das/patches/upages.diff
> 
> 
> [1] Most of the instances of PHOLD() and PRELE() right now never
>     needed to be there or have been unnecessary ever since the PCB
>     was moved out of the U area.

Go for it!  Just get some validation that amd64 and sparc64 work, and
then do the deed.  I'll try testing at least amd64 right now.

Scott