On Thu, Nov 28, 2013 at 11:40:06AM +0100, Paolo Bonzini wrote: > Il 28/11/2013 11:16, Avi Kivity ha scritto: > >> The QRCU I linked would work great latency-wise (it has roughly the same > >> latency of an rwsem but readers are lock-free). However, the locked > >> operations in the read path would hurt because of cache misses, so it's > >> not good either. > > > > I guess srcu would work. Do you know what's the typical write-side > > latency there? (in terms of what it waits for, not nanoseconds). > > If there's no concurrent reader, it's zero if I read the code right. > Otherwise it depends: > > - if there are many callbacks, only 10 of them are processed per > millisecond. But unless there are concurrent synchronize_srcu calls > there should not be any callback at all. If all VCPUs were to furiously > change the MSIs, the latency could go up to #vcpu/10 milliseconds. > > - if there are no callbacks, but there are readers, synchronize_srcu > busy-loops for some time checking if the readers complete. After a > while (20 us for synchronize_srcu, 120 us for > synchronize_srcu_expedited) it gives up and starts using a workqueue to > poll every millisecond. This should never happen unless > Unless what ? :) Unless reader is scheduled out? > So, given the very restricted usage this SRCU would have, we probably > can expect synchronize_srcu_expedited to finish its job in the > busy-looping phase, and 120 us should be the expected maximum > latency---more likely to be an order of magnitude smaller, and in very > rare cases higher. > > Paolo -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
