Kaushik Kumar Ram

Xen’s memory sharing mechanism, called the grant mecha-nism, is used to share I/O buffers in guest domains ’ mem-ory with a driver domain. Previous studies have identified the grant mechanism as a significant source of network I/O overhead in Xen. This paper describes a redesigned grant mechanism to significantly reduce the associated overheads. Unlike the original grant mechanism, the new mechanism al-lows guest domains to unilaterally issue and revoke grants. As a result, the new mechanism makes it simple for the guest OS to reduce the number of grant issue and revoke opera-tions that are needed for I/O by taking advantage of tempo-ral and/or spatial locality in its use of I/O buffers. Another benefit of the new mechanism is that it provides a unified interface for memory sharing, whether between guest and driver domains, or between guest domains and I/O devices using IOMMU hardware. We have developed an implemen-tation of the new grant mechanism that fully supports driver domains, but not yet IOMMUs. The paper presents perfor-mance results using this implementation which show that the new mechanism reduces per-packet overhead by up to 31 % and increases network throughput by up to 52%