Of the existing BucketCache IOEngines, the FileIOEngine uses pread() to copy data from kernel space to user space. This is a good choice when the total working set size is much bigger than the available RAM and the latency is dominated by IO access. However, when the entire working set is small enough to fit in the RAM, using mmap() (and subsequent memcpy()) to move data from kernel space to user space is faster. I have run some short keyval gets tests and the results indicate a reduction of 2%-7% of kernel CPU on my system, depending on the load. On the gets, the latency histograms from mmap() are identical to those from pread(), but peak throughput is close to 40% higher.
Already tested the patch at Flurry. Anyone interested in reviewing the patch? -Zee
