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buffering this protocols provides the advantage of buffering to the receiver can buffer any out of order packets after being
time outtimers are used to protect against lost packet each packet must have its own logical timers since only a
ack receivedif an ack is received by the sender the sr sender marks that packet as having been received by the
some people write custom memory allocators to meet their specic needs although this is not needed for most of the applications it is also not
how do we keep track of where the free pieces of memory are one idea is to maintain a set of linked-lists of free space each linked-list will store
since most of the programs require a lot of memory allocationdeallocation we expect the memory management to be fast to have low fragmentation make
usually memory is allocated from a large pool of unused memory area called the heap in c dynamic allocationdeallocation must be manually performed
lack of standards and experience the lack of standards has considerably limited the potential of distributed dbmss as well there are no tools or
security in a centralised system access to the data can be simply controlled though in a distributed dbms not only does access to replicated data
the question of fairness regarding page eviction is a hard one how do we decide what is fair many operating systems use global lru where pages from
complexity a distributed dbms that is available reliable and secure is inherently more difficult than a centralised dbms though adequate data
greater potential for bugs since the sites of a distributed system operate simultaneously it is more complex to ensure the correctness of algorithms
now let us discuss two related algorithms for deciding which pages to evict the clock algorithm is one of the most popular choices it works by
drawbacks of data distributionthe primary drawbacks of distributed database systems are the added complexity needed to ensure proper coordination
1 on every access mark the page with a timestamp whenever we need to evict a page we search through memory for the oldest page the least-recently
evicting the most-recently used mru page does very well on lrus worst case in general however mru is a bad idea since many programs exhibit temporal
lru evicts the page which was last accessed the farthest into the past of any page resident in physical memory ie the least-recently used page lru
the optimal replacement policy called opt is to evict the page which will be accessed farthest into the future since we cant predict the future
modular growthin distributed environments it is simple to expand latest sites can be added to the network without affecting the operations of other
the physical memory acts as a cache backed by the disk when the physical memory is full and we want to read in another page from disk we have to
economicsit is now usually accepted that it costs less to make a system of smaller computers with the equivalent power of a one large computer it is
speedup query processinga query that includes data at various sites can be split into sub-queries these sub- queries can be implemented in parallel
improved performanceas the data is located near the site of its demand and given the inherent parallelism because of multiple copies speed of
processes have valid and invalid entries on their page tables the valid entries all point to some where real eg a physical page or some portion of