David Laight <[email protected]> wrote: > > Rw locks has very bad effect on CPU caches and can slow things > > down on a bigger SMP systems. > > Actually RW locks are probably only ever worthwhile on big SMP systems! > RW locks allow additional concurrency! <...>
No, not really. Of course, RW-lock has its use and can be suitable in many cases. However, as already wrote in other email, it causes cache synchronisation among CPUs, which is relatively expensive on bigger MP systems (imagine the traffic on a system with 32 CPUs or more!). Mutual exclusion at the cache level on reader side can cause relatively strong effect (quite noticeable on recent multi-core x86 systems). For very intensive read-mostly locking, there are alternatives, such as read-optimised locks (with a high cost of write-locking), RCU-like synchronisation, lockless design or perhaps other tricks. :) -- Mindaugas
