78

User Space Memory 

Host 

Ethernet NIC 

5 

Header 

Data 

4 

NIC Memory 

Request 

6 

CPU 

Header 

8 

7 

DMA 

NIC 

3 

Controller 

Processor 

Data 

2 

1 

Request 

Kernel Space Memory 

Fig. 5.1. HTTP Processing Steps in a Distributed Web Server for Local Cache Requests

to the user space memory by the OS kernel (Step 3). The Web server reads the request

and checks whether the requested file resides in the data cache of the main memory (Step

4). In case of a cache hit, it generates the HTTP header (Step 5) and writes the header

and data into the kernel space memory (Step 6). The OS kernel transfers the header

and data into the NIC memory through DMA (Step 7). Finally, the Ethernet NIC sends

the response to the client through TCP/IP (Step 8). A local cache miss will invoke the

remote cache read, described next.

5.3.2

remote cache read

A remote cache read occurs when the node that receives a client's request from

the webswitch, called an initial node, does not have the requested file in its data cache.

In other words, the initial node has a local cache miss and thus, needs to get it from

another node, called a service node, which has the data in its local cache. The initial

node initiates a remote cache read by sending a request to a service node using backend

forwarding [13] via the intelligent NIC. Figure 5.2 depicts the processing steps at a service