* Chris Mason <chris.mason@xxxxxxxxxx> 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(-) -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
