114
5.8
Conclusions
In an effort to meet the increasing demand for Web applications and services,
clusters of Web servers interconnected by system area networks have been used to satisfy
the need cost effectively. To reduce the communication overhead, these clusters rely
on high performance, low latency intra cluster communication mechanisms, such as the
Virtual Interface Architecture (VIA) [30, 19] or InfiniBand Architecture [45]. Under these
new architectures, many programmable NICs have come with large on chip memory to
support more connections. However, this abundant NIC memory space is not fully
utilized.
In this chapter, we examine two alternative schemes that use the NIC memory
as a data cache. The exclusive caching scheme uses the NIC memory as an extended
cache for the main memory cache. This scheme increases the cache hit ratio, and thus
reduces the number of disk accesses. The exclusive scheme is particularly effective when
the number of Web data items is large and the popularity skewness is low, as in the
UCB trace. In contrast, the inclusive caching scheme maintains a copy of the most
frequently accessed data items (or files) in the NIC memory. Thus, the NIC cache is
a subset of the main memory cache. The main advantage of this scheme is to reduce
the intra cluster communication traffic and the DMA latency. The inclusive scheme is
particularly effective when the size of Web content is large and the popularity skewness
is high.
We also investigate the impact of the NIC caching schemes on performance of
cluster based Web servers using three real Web traces with different characteristics (CSE,












  

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