vmstat's entries type

[M. Warner Losh]

In message: <200607191315.k6JDFpvM048354 at lurza.secnetix.de>
            Oliver Fromme <olli at lurza.secnetix.de> writes:
: Michal Mertl <mime at traveller.cz> wrote:
:  > We had discussions about 64 bit counters several times during the years
:  > (I made a huge patch which turned every network related counter 64bit
:  > and all accesses were made with a macro) and the conclusion was that it
:  > isn't worth it.
:  > 
:  > 64bit numbers are too expensive to do correctly on 32bit machines. When
:  > done incorrectly they can easily get incorrect and that is probably
:  > worse than a simple counter in machine's native word size (which can
:  > still get incorrect on some architectures).
:  > 
:  > I expect you know that long is usually 64bit wide on 64bit
:  > architectures. The discussion about 64bit counters on 32bit machines
:  > doesn't make much sense when AMD64 is becoming the mainstream
:  > architecture and the right type to use for integers (that can get
:  > "large") is long IMHO.
: 
: I see.  I'm mostly a userland programmer, and when there's
: a variable that might overflow 32 bits, then I always use a
: 64 bit type (e.g. uint64_t), no matter whether I'm on a 32
: bit or 64 bit architecture.  It's all about portability and
: reliability.  In fact, I rarely use "long", because I think
: it's not very useful.
: 
: However, I got your point.  Kernel programming is different
: from userland programming, and I'm aware that using 64 bit
: values can cause problems on 32 bit architectures (which I
: mentioned in my previous mail).

One approach that we could use for 64-bit counters would be to just
use 32-bits one, and poll them for overflow and bump an overflow
count.  This assumes that the 32-bit counters overflow much less often
than the polling interval, and easily triples the amount of storage
for each of them...  It is ugly :-(

Warner