Jose Renato Santos
Hewlett Packard
Netchannel2:  Improving Xen Network Performance
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In this talk we present a detailed performance analysis of Xen network
virtualization and identify the major source of overheads when
processing network packets using current Xen para-virtualized network
drivers. The analysis extends the results presented in the last Xen
summit which were based on a pentium 4 plataform running a 32 bit
version of Xen, to a more modern plataform based on the Intel dual core
processor running a 64 bit Xen. While the 64 bit version of Xen reduces
some of the overheads, such as mapping domain pages in Xen space to do
grant copies, it also introduces additional overheads such as the
overhead to trap guest system calls.  The new results compare Xen and
linux in both architectures and provide additional insights into the
major sources of Xen overhead. Based on these resuts, we describe a few
optimizations that can be integrated in the current Xen para-virtualized
network drivers without any changes to the current IO channel protocol. 
We then, discuss the new IO channel protocol, netchannel2, which address
several other issues identified in our Xen network performance analysis.
We provide both an overview of the netchannel2 design and discuss how
its new features should improve network performance. We also discuss how
the current Xen grant mechanism should be extended to support the new
features of netchannel2. We finaly present experimental performance
results of a "hacked" implementation of Xen networking which simulates
the benefits of netchannel2 features including: grant reuse, lazy page
mapping for transmission, moving the receive data copy to the guest CPU,
and the use of hardware multi-queue of modern NICs to avoid the need of
copying data in the receive path. Although a netchannel2 implementation
is not available yet, we have confidence that our experimental results
accurately estimates the performance improvements that will be achieved
by netchannel2.