Re: [PATCH -v4][RFC]: mutex: implement adaptive spinning

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

On Wed, 2009-01-07 at 09:50 -0800, Linus Torvalds wrote:
> 
> On Wed, 7 Jan 2009, Chris Mason wrote:
> > 
> > So far I haven't found any btrfs benchmarks where this is slower than
> > mutexes without any spinning.  But, it isn't quite as fast as the btrfs
> > spin.
> 
> Quite frankly, from our history with ext3 and other filesystems, using a 
> mutex in the filesystem is generally the wrong thing to do anyway. 
> 
> Are you sure you can't just use a spinlock, and just release it over IO? 
> The "have to do IO or extend the btree" case is usually pretty damn clear.
> 
> Because it really sounds like you're lock-limited, and you should just try 
> to clean it up. A pure "just spinlock" in the hotpath is always going to 
> be better.

There are definitely ways I can improve performance for contention in
the hot btree nodes, and I think it would be a mistake to tune the
generic adaptive locks just for my current code.

But, it isn't a bad test case to compare the spin with the new patch and
with the plain mutex.  If the adaptive code gets in, I think it would be
best for me to drop the spin.

Either way there's more work to be done in the btrfs locking code.

-chris