Messages in this thread | | | Date | Fri, 3 Nov 2006 12:02:34 -0800 | From | "Paul E. McKenney" <> | Subject | Re: New filesystem for Linux |
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On Thu, Nov 02, 2006 at 03:15:33PM -0800, Linus Torvalds wrote: > > > On Thu, 2 Nov 2006, Mikulas Patocka wrote: > > > > * There is a rw semaphore that is locked for read for nearly all operations > > and locked for write only rarely. However locking for read causes cache line > > pingpong on SMP systems. Do you have an idea how to make it better?
[ . . . ]
> (Seqlocks could be changed to drop the first requirement, although it > could cause some serious starvation issues, so I'm not sure it's a good > idea. For RCU the atomic nature is pretty much designed-in.)
I can't help putting in a plug for SRCU, which is in the 2.6.19-rc series, and which allows readers to sleep. (http://lwn.net/Articles/202847/)
SRCU allows readers and writers to run concurrently (as do all forms of RCU). If this is a problem, it might be worth looking into Gautham Shenoy's reader-writer lock built on top of RCU. (A version for hotplug may be found at http://lkml.org/lkml/2006/10/26/73.) This approach keeps the reader-writer-lock semantics, but gets rid of cache thrashing. That said, writers have to wait for a grace period.
And as Linus pointed out, if you have disk I/O involved, you probably won't notice normal reader-writer-lock overhead.
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