Re: Btrfs for mainline

Matthew Wilcox
2009-01-04T18:41:03Z

On Sun, Jan 04, 2009 at 07:21:50PM +0100, Peter Zijlstra wrote:
> The -rt tree has adaptive spin patches for the rtmutex code, its really
> not all that hard to do -- the rtmutex code is way more tricky than the
> regular mutexes due to all the PI fluff.
>
> For kernel only locking the simple rule: spin iff the lock holder is
> running proved to be simple enough. Any added heuristics like max spin
> count etc. only made things worse. The whole idea though did make sense
> and certainly improved performance.

That implies moving

struct thread_info *owner;

out from under the CONFIG_DEBUG_MUTEXES code. One of the original
justifications for mutexes was:

- 'struct mutex' is smaller on most architectures: .e.g on x86,
'struct semaphore' is 20 bytes, 'struct mutex' is 16 bytes.
A smaller structure size means less RAM footprint, and better
CPU-cache utilization.

I'd be reluctant to reverse that decision just for btrfs.

Benchmarking required! Maybe I can put a patch together that implements
the simple 'spin if it's running' heuristic and throw it at our
testing guys on Monday ...

--
Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."