How to flush the disk read cache under Windows? [duplicate]

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How to invalidate the file system cache?

In order to be able to so some benchmarks I need to cleanup Windows disk read cache. How can I do this?

In fact I want to compare if loading a big Unicode file (UTF-8 or UTF-16) from disk is faster or not, considering that in memory I do keep UTF-16.

I know that it should be no significant difference but in order to benchmark it I need to be sure that that file is not cached - I need to see if size on disk has more or less impact than decoding the file.

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Someone from Microsoft helped me out with this a few years ago (back in the days of Windows XP) and gave me a solution that met my needs at the time. I already had two drives in my machine, and what I needed to test was already on the D drive, and fortunately I didn't need to keep any files open on my D drive when I wanted to flush the cache. From the Windows Disk Management interface, I could change my D: drive letter to another letter, and then back again, and the D drive would perform as if had a cold cache in the OS.

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AFAIK, it's unfortunately not possible to discard the read cache under Windows. I spent some time looking into this some years ago, and only found out how to flush the write cache.

As I see it, you have three options, unless somebody else has found some magic:

1. If possible, do your read file I/O in unbuffered mode.
2. Each time you want to benchmark, create a new copy of the test data specifying unbuffered mode when creating the new copy (this should keep the copy out of read cache, but I haven't tested).
3. Allocate enough memory that windows has to discard the disk cache (ugh!).

EDIT: it is indeed possible to flush the read cache, at least on Vista and later: Disable or flush page cache on Windows. It requires a call to the undocumented NtSetSystemInformation NT API, though. Also, for a single file, read cache can be flushed simply by opening the file with FILE_FLAG_NO_BUFFERING specified and closing the handle again.

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The only solution I found so far was http://chadaustin.me/2009/04/flushing-disk-cache/ but this ones takes too much time so I hope we'll find a better one.