On 08/07/2013 06:40 AM, Jan Kara wrote:

  One question before I look at the patches: Why don't you use fallocate()
in your application? The functionality you require seems to be pretty
similar to it - writing to an already allocated block is usually quick.

One problem I've seen is that it still costs you a fault per-page to get
the PTEs in to a state where you can write to the memory.  MADV_WILLNEED
will do readahead to get the page cache filled, but it still leaves the
pages unmapped.  Those faults get expensive when you're trying to do a
couple hundred million of them all at once.

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