Modern IA-32 processors support a physical address extension (PAE) mode that allows the hardware to address up to 64 GB of memory with 36-bit addresses . However, many devices that use DMA for I/O transfers can address only a subset of this memory. For example, some network interface cards with 32-bit PCI interfaces can address only the lowest 4 GB of memory.
Some high-end systems provide hardware support that can be used to remap memory for data transfers using a separate I/O MMU. More commonly, support for I/O involving ``high'' memory above the 4 GB boundary involves copying the data through a temporary bounce buffer in ``low'' memory. Unfortunately, copying can impose significant overhead resulting in increased latency, reduced throughput, or increased CPU load.
This problem is exacerbated by virtualization, since even pages from virtual machines configured with less than 4 GB of ``physical'' memory may be mapped to machine pages residing in high memory. Fortunately, this same level of indirection in the virtualized memory system can be exploited to transparently remap guest pages between high and low memory.
ESX Server maintains statistics to track ``hot'' pages in high memory that are involved in repeated I/O operations. For example, a software cache of physical-to-machine page mappings (PPN-to-MPN) associated with network transmits is augmented to count the number of times each page has been copied. When the count exceeds a specified threshold, the page is transparently remapped into low memory. This scheme has proved very effective with guest operating systems that use a limited number of pages as network buffers. For some network-intensive workloads, the number of pages copied is reduced by several orders of magnitude.
The decision to remap a page into low memory increases the demand for low pages, which may become a scarce resource. It may be desirable to remap some low pages into high memory, in order to free up sufficient low pages for remapping I/O pages that are currently ``hot.'' We are currently exploring various techniques, ranging from simple random replacement to adaptive approaches based on cost-benefit tradeoffs.