AIX 3.1 Kernel Size

[John F Haugh II]

In article <479 at ulticorp.UUCP> rick at ulticorp.UUCP (Rick Poleshuck) writes:
>In article <19039 at rpp386.cactus.org> jfh at rpp386.cactus.org (John F Haugh II) writes:
>>Yes, it is "correct", however, it is a paging kernel and much of that
>>space is large tables that are unused until you need them.  It doesn't
>>mean the same thing it does with any other UNIX-derivative.
>
>I am confused. We have a Model 520 with 16Meg of ram. Our size size
>command also shows 20.5 Mb of .BSS ( uninitialized data space ). Even
>the Unix kernel can't be using 20 megabytes of tables. I assume that
>this memory MUST be used for disk cache buffers. Since real memory is
>less than the buffer size AIX is paging cache buffers to disk?

I understand your confusion - AIX v3 is either the only UNIX-like kernel
that pages or one of very few.

There are various tables, the process table (struct proc) is one of them,
that are declared as having very large sizes.  If you check out the size
of the process table you will see that it is very large - on the order
of 262,144 entries (I forget the exact value).  All of that space is in
the .bss segment.  Since the kernel pages its own text and data pages,
the .bss segment doesn't exist until needed - the same as your user mode
program.  When a process table entry is referenced and the entry does not
exist, the system will catch a kernel mode page fault and allocate a page
to the process table.

Even many parts of the .text and .data segments are pagable.  It is quite
possible for a fairly small amount of the kernel to be "pinned" at any
given time.  So, when I said that the "size" command on the AIX kernel
doesn't mean anything particularly useful, I mean you have to thoroughly
understand how the system allocates it's memory and fully understand that
the size of the system as reported by various utilities is pretty
meaningless.
-- 
John F. Haugh II                             UUCP: ...!cs.utexas.edu!rpp386!jfh
Ma Bell: (512) 832-8832                           Domain: jfh at rpp386.cactus.org
"I've never written a device driver, but I have written a device driver manual"
                -- Robert Hartman, IDE Corp.