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    <p>1 is definitely correct. The operating system can fetch from the disk to the cache while your code is processing the data it's already received. Yes, the disk may well still be the bottleneck - but you won't have read, process, read, process, read, process, but read+process, read+process, read+process. For example, suppose we have processing which takes half the time of reading. Representing time going down the page, we might have this sort of activity without prefetching:</p> <pre><code>Read Read Process Read Read Process Read Read Process </code></pre> <p>Whereas with prefetching, this is optimised to:</p> <pre><code>Read Read Read Process Read Read Process Read Process </code></pre> <p>Basically the total time will be "time to read whole file + time to process last piece of data" instead of "time to read whole file + time to process whole file".</p> <p>Testing it is tricky - you'll need to have an operating system where you can tweak or turn off the cache. Another alternative is to change how you're opening the file - for instance, in .NET if you open the file with <a href="http://msdn.microsoft.com/en-us/library/system.io.fileoptions.aspx" rel="nofollow noreferrer">FileOptions.SequentialScan</a> the cache is more likely to do the right thing. Try with and without that option.</p> <p>This has spoken mostly about prefetching - general caching (keeping the data even after it's been delivered to the application) is a different matter, and obviously acts as a big win if you want to use the same data more than once. There's also "something in between" where the application has only requested a small amount of data, but the disk has read a whole block - the OS isn't actively prefetching blocks which haven't been requested, but can cache the whole block so that if the app then requests more data from the same block it can return that data from the cache.</p>
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    1. POA question about cache of filesystem
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    1. CODo you sleep? ;-)
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    2. COYes, but now I have to get my eldest son changed after swimming. Back later :)
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    3. CO@Jon Skeet..:But is the time to process the last piece of data comparable with the time to prefetch the next piece of data? And another questions is :If I turn off the cache and randomly access the disk, is the time needed comparable with the time when sequentially read with the cache turned on?
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