77
5.3
Communication mechanisms in a Cluster Based Web server
Generally, each node in cluster based Web servers has two NICs; an Ethernet 
based NIC and an intelligent NIC. The Ethernet based NIC is used to communicate
with clients outside the cluster through TCP/IP, while the high performance intelligent
NIC is used for the intra cluster communication. Although the Ethernet NIC can be used
for both outside and intra cluster communication, this may degrade system performance
due to the TCP/IP overhead. Especially, high network latency of an Ethernet NIC can
negate the benefit of the backend forwarding scheme [13]. Thus, a low latency NIC is
essential for handling the intra cluster communication.
In a cluster based distributed Web server, like PRESS [23], there are three ways
to fetch data content for serving a user's request: local disk read, local cache read, and
remote cache read. The local disk read, serves the requests by finding data or files from
its local disk, is the bottom line approach due to the expensive disk I/O cost. Since
the focus of the chapter is to use cache to improving system performance, we explain in
detail the steps of the local cache read and the remote cache read operations.
5.3.1
local cache read
A local cache read occurs when a Web server can simply fetch the requested file
from its local memory. Figure 5.1 depicts the steps in processing an HTTP request
through the TCP/IP protocol in a distributed Web server. The communication is initi 
ated when an Ethernet NIC receives a request from a client (Step 1) and the request is
written into the kernel space memory using DMA (Step 2). Next, this request is passed












  

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