vmstat's entries type
Brian Candler
B.Candler at pobox.com
Fri Jul 28 12:15:31 UTC 2006
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.
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