Re: vmstat's entries type

On Tuesday 25 July 2006 08:54, Oliver Fromme wrote:
> John Baldwin <jhb_at_freebsd.org> wrote:
>  > On Sunday 23 July 2006 20:03, Sten Daniel Sørsdal wrote:
>  > > sthaug_at_nethelp.no wrote:
>  > > > > > 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 :-(
>  > > > > > 
>  > > > > What's wrong with the add+adc (asm) approach found on any i386?
>  > > > 
>  > > > Presumably the fact that add + adc isn't an atomic operation. So if
>  > > > you want to guarantee 64 bit consistency, you need locking or 
similar.
>  > > > 
>  > > 
>  > > Would it not be necessary to do this locking anyway?
>  > > I don't see how polling for overflow would help this consistency.
>  > > Are both suggestions insufficient?
>  > 
>  > I actually think that add + adc is ok for the case of incrementing simple 
>  > counters.  You can even do 'inc ; addc $0'
> 
> (I'm familiar with asm programming, but I'm not a low-level
> threading or SMP expert, so please excuse me if this is a
> dumb question ...)
> 
> If you just do add+adc (or inc+adc) and another thread (on
> the same or different processor, I don't know) happens to
> read the counter value at the same time (i.e. after the
> lower 32bit have overflowed, but before the upper 32bit get
> incremented), then that other thread would get a value
> that's off by 2^32.
> 
> What am I missing?

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.

-- 
John Baldwin