Re: [PATCH] 2.6.x BSD Process Accounting w/High UID

From: Albert Cahalan
Date: Wed Mar 10 2004 - 11:38:18 EST

On Wed, 2004-03-10 at 04:08, Tim Schmielau wrote:
> On Wed, 9 Mar 2004, Albert Cahalan wrote:
>> First of all, none of this matters much if the format
>> is given a sysctl. The old format is default for now,
>> and the new one is default (only?) in a couple years.
>> Sun appears to have done something like this.
> Yeah, if we even want to introduce a new syscall for this...
> But Documentation/Changes is not an empty file, i.e., we
> require people to upgrade userspace anyways.

In that case, don't forget to upgrade atop.
That's a version of top that uses BSD process
accounting to grab the transient processes
that wouldn't be seen in /proc before they die.

>> When fixing it, note that a 5-bit binary exponent
>> with denormals would beat the current float format.
> Yes, but only by a short head.

It's by 8 bits, with a stable 11-bit fraction
instead of a 10-bit to 12-bit variable-size one.

old: 1xxxxxxxxxxyy000000000000000000000
new: 1xxxxxxxxxxx000000000000000000000000000000

That's a 42-bit number instead of a 36-bit one.
The old base-8 exponent is wasteful, plus the old
format stores the leading 1 instead of using an
implied 1 and special exponent for leading 0.

> And, since comp_t is hard-coded into
> current GNU acct tools, we can't keep source compatibility
> (not that this matters too much anyways...)
> I'm open for suggestions in this direction. Any reasonable ideas to
> get more precision? (e.g. 16 bit mantissa and 4 bit base-whatever
> exponent?)

a. just what I said
b. 32-bit IEEE float (easy enough to encode by hand)
c. raw data -- is the space saving all that critical?
d. raw data with gzip-style compression

(note that gzip's deflate algorithm is in the kernel)

> > Regarding the existing struct though...
> >
> > Let's take a close look at this. I think there are 2 bytes
> > of padding on all Linux ports, and another 2 available
> > on everything except maybe m68k and/or arm. (that is, ports
> > that will put a u32 on any u16 boundry) Here is the current
> > struct, compactly formatted with 64-bit blocking:
> >
> > struct linux_acct {
> > char ac_flag; // Flags
> > // 1 pad byte
> Yep, but depending on architecure I think the compiler is free to insert
> the padding either before or after ac_flags.

I doubt it. I think the C standard has something to
say about this. In any case, I just checked a mix of
big-endian and little-endian boxes:

32-bit BE SPARC
64-bit BE SPARC
32-bit BE PowerPC
32-bit LE i386
64-bit LE x86-64
64-bit LE Alpha

In every case, 1-byte padding came after ac_flag.

I'm pretty sure ac_comm is too big as well. It has
room for 17 bytes. It needs room for 15, plus a '\0'
if you want one.

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