Menu: Backend

Introduction to Prefetching Feature

Loading of data from slow persistent memory (disk storage) to main memory represents a bottleneck for current interactive visual data exploration applications, especially when applied to huge volume of data. Semantic caching of queries at the client-side can significantly improve the performance of such interactive applications. We hence propose to augment the semantic caching techniques by applying prefetching. The system predicts the user’s next requested data and loads the data into the cache as a background process before the next user request is made. However, a given pre-fetching technique is not always able to correctly predict changes in a user’s navigation pattern. Especially, as different users may have different navigation patterns, implying that the same strategy might fail for a new user. Thus for utilizing the adaptation, we introduce a concept of strategy selection to allow the choice of pre-fetching strategy to change over time both across as well as within one user session.

At present, Pre-fetching Feature only work in hierarchical view.

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Random Strategy

As shown in Figure 4.2, strategy S1 (random) is based on randomly choosing the direction in which to prefetch next. The directions are either lateral (left or right at the same level in hierarchy) or vertical (increase or decrease level of detail). Our visualization tool only allows manipulation in one of those four directions (using six possible operations).

This strategy is appropriate when the predictor either cannot extract prefetching hints or provides hints with a low confidence measure.

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Direction Strategy

Strategy S2 (direction) is analogous to the sequential prefetching scheme. This direction strategy assumes that the most likely direction of the next operation can be determined. It is intuitive, for instance, that the user will continue to use the same navigation tool for a while before changing to other one. In our system, each navigation tool of the structure-based brush happens to precisely control one direction only. Based on a user’s past explorations, the predictor would assign probabilities to all the four directions. The prefetching strategy (S2) then is to “prefetch data in the direction” currently with the highest probability.

As depicted in Figure 4.3, if (m-1) and m are the last two directions navigated into by the user, then he direction strategy may predict (m+1) as the next direction to be visited by the user in the same direction of the previous two movements.

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Focus Strategy

Strategy S3 (focus) uses information about the most probable next direction (by keeping track of user’s previous movement) as well as hints about regions of high interest (hot regions, as depicted in Figure 4.4) in the data space as identified based on prior navigations of this same data by other users. We found the hot regions for each user by keeping the statistics of the regions visited by the user during the past explorations, and then maintaining all the regions that have frequency of visits above a particular threshold as hot regions. This strategy will continue to prefetch data in the given direction using the above mentioned direction heuristics. However, when a hot region is around the current navigation window, the prefetcher switches from the default direction prefetching to prefetch in that now more desirable direction. The hypothesis is that the user will likely stop at such a region of interest to explore those hot regions only if she got close enough to notice it.

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Command - Connect Database:

Function:

Connect to database and enable prefetching feature.

Operations:

Tips:

          If connection succeeds, "Connected to database successfully" message will be shown in status bar.

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Command - Disconnect Database:

Function:

Disconnect to database and disable prefetching feature.

Operation:

Tips:

After disconnect to database, Xmdv Tool will automatically use data from file.

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Command - Prefetcher:

Function:

It is not used in this version.  Prefetching Feature will be enabled after connection to database.

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Command - Prefetcher Strategy:

Function:

Select one Prefetcher Strategy from the list: No Prefetcher, Random, Direction and Focus.

Operation:

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