Re: Btrfs for mainline

Ingo Molnar
2009-01-07T13:07:42Z

* Chris Mason <chris.mason@oracle.com> wrote:

> On Tue, 2009-01-06 at 01:47 +1100, Nick Piggin wrote:
>
> [ adaptive locking in btrfs ]
>
> > adaptive locks have traditionally (read: Linus says) indicated the locking
> > is suboptimal from a performance perspective and should be reworked. This
> > is definitely the case for the -rt patchset, because they deliberately
> > trade performance by change even very short held spinlocks to sleeping locks.
> >
> > So I don't really know if -rt justifies adaptive locks in mainline/btrfs.
> > Is there no way for the short critical sections to be decoupled from the
> > long/sleeping ones?
>
> Yes and no. The locks are used here to control access to the btree
> leaves and nodes. Some of these are very hot and tend to stay in cache
> all the time, while others have to be read from the disk.
>
> As the btree search walks down the tree, access to the hot nodes is best
> controlled by a spinlock. Some operations (like a balance) will need to
> read other blocks from the disk and keep the node/leaf locked. So it
> also needs to be able to sleep.
>
> I try to drop the locks where it makes sense before sleeping operatinos,
> but in some corner cases it isn't practical.
>
> For leaves, once the code has found the item in the btree it was looking
> for, it wants to go off and do something useful (insert an inode etc
> etc). Those operations also tend to block, and the lock needs to be held
> to keep the tree block from changing.
>
> All of this is a long way of saying the btrfs locking scheme is far from
> perfect. I'll look harder at the loop and ways to get rid of it.

<ob'plug>

adaptive spinning mutexes perhaps? Such as:

http://lkml.org/lkml/2009/1/7/119

(also pullable via the URI below)

If you have a BTRFS performance test where you know such details matter
you might want to try Peter's patch and send us the test results.

Ingo

------------->

You can pull the latest core/locking git tree from:

git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip.git core/locking

------------------>
Peter Zijlstra (1):
mutex: implement adaptive spinning

include/linux/mutex.h | 4 +-
include/linux/sched.h | 2 +
kernel/mutex-debug.c | 10 +------
kernel/mutex-debug.h | 8 -----
kernel/mutex.c | 46 ++++++++++++++++++++++--------
kernel/mutex.h | 2 -
kernel/sched.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++
kernel/sched_debug.c | 2 +
kernel/sched_features.h | 1 +
9 files changed, 115 insertions(+), 33 deletions(-)