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 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
