Results

 

To make it easier to interpret the results, we sorted URLs by the average uncompressed file size. Figure 7 graphs the average size (across all versions of a page) as a function of the sorted URL numbers, which are used in the other graphs below.

  

Figure 7: Average uncompressed data size across all versions of each page, sorted by size, shown on a log scale.

Figure 8 shows our results. Each graph plots the average ratio of end-to-end latency using the modified system to latency using the unmodified system. The left column shows cases where the client proxy caches the previous version (simple deltas), while the right column shows the use of optimistic deltas. The first row shows no added content provider latency, and the second row shows 5s of added latency. The URLs are sorted in the same sequence as in Figure 7. The solid line in each graph indicates the mean of all the points in the graph, while the dashed line indicates the break-even point.

The cost of computing deltas and patching was negligible (1-2%) compared to the network transfer time and protocol processing overhead in all our experiments. Moreover, the largest measured value of overhead from computing a delta and applying the delta on the client was much less than the typical variation in the total URL fetch times.

 

 

Figure 8: Experimental results, showing ratios of end-to-end latency for modified versus unmodified system, varying whether old versions are cached on the client or sent optimistically by the server, and whether the content provider adds 0s or 5s of latency before returning content. Each data point represents the average across all versions for the corresponding page. The solid line in each graph indicates the mean of all the points in the graph, while the dashed line indicates the break-even point. The ``simple delta'' case never experienced aborts, while the ``optimistic delta'' case experienced aborts that are indicated with a different symbol.

From Figure 8 we draw the following conclusions:

• The pages with the lowest index, which have the largest original file size, tended to show more improvement than the smaller pages, but although the general trend is upward as one moves right along the X-axis, there are great variations from page to page.
• As expected, without added latency, many of the pages took longer using the optimistic approach than without it. The measurements in Figure 8(b) were taken with a simple abort strategy in place. This strategy aborted only when the server proxy had finished computing the delta and the amount of remaining stale data plus the delta was more than the size of the regular response. We expect that a smarter abort strategy, such as aborting an ongoing optimistic transfer of stale data and ``cutting through'' new data even as it is being received if it appears that it is very different from the cached stale data, would cap the latency of our system at close to 100% of the unmodified system.
• With 5s added latency, most pages were received faster by the client using optimistic deltas, with a mean improvement of 27%. In fact, the latency for optimistic deltas with 5s added delay was consistently somewhat less than that for simple deltas with the same delay. We attribute the better performance of optimistic deltas to TCP's slow-start algorithm [13]. In the case of the optimistic deltas the transfer of the stale data opened up the TCP congestion window, so the deltas were transferred faster in this case than in the case of simple deltas, where the transfer of the delta had to open up the congestion window itself.
• Nearly all of the simple deltas improved performance regardless of added latency, which one would expect. As predicted, the relative gain was generally better when the fixed overhead was lower. In the case of 0s added latency, most of the points that showed degradation were cases of only two versions being available (hence a greater likelihood of variability due to external factors) and where the deltas were 40-60% of the original file size. The overall improvement was 33%, which was the best of the four configurations.