Anomalous performance increase from mutex profiling

[Kris Kennaway]

On Mon, Apr 17, 2006 at 03:54:07AM -0400, Surer Dink wrote:
> On Mon, 17 Apr 2006, Kris Kennaway wrote:
> 
> > On Mon, Apr 17, 2006 at 01:14:40AM -0400, Kris Kennaway wrote:
> >
> >> * Our best guess is that mutex profiling is doing something that
> >> reduces contention on this very heavily contended mutex (unp), but I'd
> >> like to know what is happening precisely so I can maybe make use of
> >> it.
> >>
> >> Can anyone think of what may be happening that I've missed?
> >
> > I think it is just doing effectively the same thing as my exponential
> > spin backoff patch, namely introducing delays with the effect of
> > reducing common memory accesses.  When I turn the maximum spin backoff
> > limit *way* up (from 1600 to 51200) I get performance that slightly
> > exceeds what I see from mutex profiling alone (adding mutex profiling
> > again on top of this gives a small further increase, but only a few %
> > and so probably achievable by further increasing the backoff limit).
> >
> > A limit of 51200 is not an appropriate default since it penalizes the
> > common case of light to moderate contention.  The point is that here
> > basically all 12 CPUs are spinning on a single lock
> > (kern/uipc_usrreq.c:308), so it's massively over-contended and all we
> > can do is mitigate the effects of this.
> >
> > On this system, the maximum supersmack performance (3700 queries/sec)
> > comes when there are only 6 clients, so (as jasone eloquently put it)
> > with 10 clients the difference between 2300 queries/sec (with absurdly
> > high backoff limits or mutex profiling) and 1450/sec (with reasonable
> > backoff limits) is the difference between "slow" and "ass slow".
> 
> Please excuse if this is a stupid question - but might using MCS or
> QOLB locks in this situation be useful?

What are they?

Kris

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