tailieunhanh - Building Secure and Reliable Network Applications phần 4

Theo quan điểm này, người ta sẽ mô tả một hệ thống máy tính về các thiết lập của các đối tượng mà nó được lắp ráp, cùng với các quy tắc mà theo đó các đối tượng tương tác với nhau. Thiết kế hệ thống hướng đối tượng đã trở thành một chủ đề chính cho nghiên cứu, | Chapter 8 Operating System Support for High Performance Communication 155 U-Net maps the communication segment of a process directly into its address space pinning the pages into physical memory and disabling the hardware caching mechanisms so that updates to a segment will be applied directly to that segment. The set of communication segments for all the processes using U-Net is mapped to be visible to the device controller over the I O bus of the processor used the controller can thus initiate DMA or direct memory transfers in and out of the shared region as needed and without delaying for any sort of setup. A limitation of this approach is that the I O bus is a scarce resource shared by all devices on a system and the U-Net mapping excludes any other possible mapping for this region. However some machines for example on the cluster-style multiprocessors discussed in Chapter 24 there are no other devices contending for this mapping unit and dedicating it to the use of the communications subsystem makes perfect sense. The communications segment is directly monitored by the device controller. U-Net accomplishes this by reprogramming the device controller although it is also possible to imagine an implementation in which a kernel driver would provide this functionality. The controller watches for outgoing messages on the send queue if one is present it immediately sends the message. The delay between when a message is placed on the send queue and when sending starts is never larger than a few microseconds. Incoming messages are automatically placed on the receive queue unless the pool of memory is exhausted should that occur any incoming messages are discarded silently. To accomplish this U-Net need only look at the first bytes of the incoming message which give the ATM channel number on which it was transmitted. These are used to index into a table maintained within the device controller that gives the range of addresses within which the communications segment can

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