Re: btrfs_tree_lock & trylock

[Chris Mason <chris.mason@oracle.com>]

On Mon, 2008-09-08 at 08:07 -0700, Stephen Hemminger wrote:
> On Mon, 8 Sep 2008 16:20:52 +0200
> Andi Kleen <andi@firstfloor.org> wrote:
> 
> > On Mon, Sep 08, 2008 at 10:02:30AM -0400, Chris Mason wrote:
> > > On Mon, 2008-09-08 at 15:54 +0200, Andi Kleen wrote:
> > > > > The idea is to try to spin for a bit to avoid scheduling away, which is
> > > > > especially important for the high levels.  Most holders of the mutex
> > > > > let it go very quickly.
> > > > 
> > > > Ok but that surely should be implemented in the general mutex code then
> > > > or at least in a standard adaptive mutex wrapper? 
> > > 
> > > That depends, am I the only one crazy enough to think its a good idea?
> > 
> > Adaptive mutexes are classic, a lot of other OS have it.
> 
> The problem is that they are a nuisance. It is impossible to choose
> the right trade off between spin an no-spin, also they optimize for
> a case that doesn't occur often enough to be justified.
> 
> People seem to repeatedly come up with adaptive mutex based on intuitive
> hunch, and never do much analysis before or afterwards.
> 

In my case, it very easy to measure.  Just watch the context switch rate
on any metadata intensive workload.  The current code scores higher on
benchmarks and uses less system time overall.

-chris