U Area Removal

John Baldwin jhb at FreeBSD.org
Thu Nov 11 09:29:02 PST 2004

On Wednesday 10 November 2004 10:00 pm, 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.

Yay!  Two notes:

+struct pstats *
+	return (malloc(sizeof(struct plimit), M_SUBPROC, M_ZERO|M_WAITOK));

I think you mean s/plimit/pstats/ there.


- * Per process structure containing data that isn't needed in core
- * when the process isn't running (esp. when swapped out).
+ * Formerly the per-process structure containing data that isn't needed
+ * in core when the process is swapped out, this structure remains only
+ * for the benefit of a.out core dumps.

s/out, this/out.  This/

It's a whole new sentence. :)

John Baldwin <jhb at FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org

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