Re: vmstat's entries type

On Thu, Jul 27, 2006 at 10:58:12AM -0400, John Baldwin wrote:
> That these counters are for stats. :)  You always have a race when reading the 
> amount, so you can choose what is "good enough" to satisfy the conflicting 
> requirements of "cheap" and "accurate".  To me, the cheapness of add+adc 
> (compared to say, a cmpxchg8b loop with a branch, etc.) is worth it if you 
> have this rare race.

You can work around the problem when reading - e.g. read twice and check the
values are close.

But is add + adc safe for update? What about the following:

- processor 1 reads low32 as FFFFFFFF
- processor 2 reads low32 as FFFFFFFF
- processor 1 writes low32 as 00000000 and sets carry
- processor 2 writes low32 as 00000000 and sets carry
- processor 1 adds 1 to high32
- processor 2 adds 1 to high32

I'm not saying this sequence can definitely occur - I'm thinking from a
general point of view, and I don't know the i386 instruction set. It just
seems plausible.

OTOH, if the above race can occur, it would imply that even a simple 32-bit
counter update could lose counts.

Regards,

Brian.