Messages in this thread | | | Date | Fri, 22 Aug 2008 10:06:02 +0300 | From | "Pekka Enberg" <> | Subject | Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather than rcu |
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Hi Ingo,
On Fri, Aug 22, 2008 at 9:28 AM, Ingo Molnar <mingo@elte.hu> wrote: > > * Jeremy Fitzhardinge <jeremy@goop.org> wrote: > >> RCU can only control the lifetime of allocated memory blocks, which >> forces all the call structures to be allocated. This is expensive >> compared to allocating them on the stack, which is the common case for >> synchronous calls. >> >> This patch takes a different approach. Rather than using RCU, the >> queues are managed under rwlocks. Adding or removing from the queue >> requires holding the lock for writing, but multiple CPUs can walk the >> queues to process function calls under read locks. In the common >> case, where the structures are stack allocated, the calling CPU need >> only wait for its call to be done, take the lock for writing and >> remove the call structure. >> >> Lock contention - particularly write vs read - is reduced by using >> multiple queues. > > hm, is there any authorative data on what is cheaper on a big box, a > full-blown MESI cache miss that occurs for every reader in this new > fastpath, or a local SLAB/SLUB allocation+free that occurs with the > current RCU approach?
Christoph might have an idea about it.
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