[Libguestfs] [PATCH nbdkit 0/2] allocators: sparse: Split the highly contended mutex

nbdkit-memory-plugin is a RAM disk, but it is quite a slow one because of the lock we must acquire on the sparse array. Try to improve performance by changing this mutex for a read-write lock. The results are somewhat mixed. Note that in the real world, workloads which constantly update the metadata are assumed to be fairly rare. The test program added in the first patch exercises metadata updates a lot more aggressively than would be expected. Rich.